CMiq: Considering that we're talking about the Creation Engine here "2 or 3 crashes" over the span of 12 hours can be considered "stable and flawlessly".

JomVanFliet: Requires that you own the GOG version of Fallout 4:
https://www.gog.com/game/fallout_4_game_of_the_year_edition

and that we're going to do a clean, from scratch and completely untouched installation of both Fallout 4 and Fallout: London. Meaning you'll have to make sure that there are no remnants of previous installation attempts still present on your machine.

We also won't be using the optional Galaxy client at any point, so if you have it up and running, shut it down.

Downloading the offline installers

1. While logged into the GOG website (https://www.gog.com) go into your library (https://www.gog.com/account)

2. Bring up the entry for Fallout 4: https://i.imgur.com/clarF4Z.jpeg

3. Click on it to expand: https://i.imgur.com/iTZOn9L.jpeg

4. Ignore the big blue "Download and Install Now", click on "Download Offline Backup Game Installers" instead to expand: https://i.imgur.com/mcLVsIq.jpeg

5. Ignore the installers for the "High Resolution Texture Pack" a little further down and download all 10 files ("Fallout 4: Game of the Year Edition (Part 1 of 10)" to "Fallout 4: Game of the Year Edition (Part 10 of 10)") to a folder somewhere on your drive: https://i.imgur.com/fSLigWo.jpeg (may have to change a certain visibility setting to have the .bin files show)

6. Make sure that all files have been downloaded correctly (check the file sizes) and are located in the same folder

7. Repeat the process for Fallout: London:
https://i.imgur.com/m35gCWh.jpeg
https://i.imgur.com/bzBeW5F.jpeg
https://i.imgur.com/iMOZGHx.jpeg

Installing Fallout 4 & Fallout: London

1. Navigate to the folder containing the 10 files for Fallout 4 (1x .exe & 9x .bin files) downloaded earlier - again make sure that all of them are present in the same folder and have been downloaded correctly

2. Double-click setup_fallout_4_game_of_the_year_edition_1.10.163.0_(64bit)_(66700).exe, and follow the install instructions

3. Once it's done installing, close the installation wizard

4. Repeat the the process for Fallout: London

Integrate Fallout: London into/Let Fallout: London take over Fallout 4

1. Navigate to the folder you have Fallout: London installed in and double-click installer.exe: https://i.imgur.com/gEqJrgq.jpeg

2. Follow the install instructions (it should automatically detect the location where you have Fallout 4 installed in, double-check if it's the correct path under "Fallout: London files will be installed in:", click "Change location", if necessary)

3. Once it's done installing/copying/integrating, close the installation wizard

Redundant since Buffout 4 already is and will be included with Fallout: London from v1.01 and onwards [̶b̶]̶I̶n̶s̶t̶a̶l̶l̶i̶n̶g̶ ̶B̶u̶f̶f̶o̶u̶t̶ ̶4̶ ̶&̶ ̶x̶S̶E̶ ̶P̶l̶u̶g̶i̶n̶ ̶P̶r̶e̶l̶o̶a̶d̶e̶r̶[̶/̶b̶]̶

̶A̶s̶ ̶d̶e̶s̶c̶r̶i̶b̶e̶d̶ ̶[̶u̶r̶l̶=̶"̶h̶t̶t̶p̶s̶:̶/̶/̶w̶w̶w̶.̶g̶o̶g̶.̶c̶o̶m̶/̶f̶o̶r̶u̶m̶/̶f̶a̶l̶l̶o̶u̶t̶_̶l̶o̶n̶d̶o̶n̶/̶f̶a̶l̶l̶o̶u̶t̶_ ̶l̶o̶n̶d̶o̶n̶_̶q̶u̶i̶c̶k̶_̶s̶t̶a̶r̶t̶_̶g̶u̶i̶d̶e̶_̶c̶o̶m̶m̶o̶n̶_̶b̶u̶g̶_̶f̶i̶x̶e̶s̶/̶p̶o̶s̶t̶1̶"̶]̶h̶e̶r̶e̶[̶/ ̶u̶r̶l̶]̶:̶

̶-̶ ̶M̶a̶n̶u̶a̶l̶l̶y̶ ̶d̶o̶w̶n̶l̶o̶a̶d̶ ̶[̶u̶r̶l̶=̶"̶h̶t̶t̶p̶s̶:̶/̶/̶w̶w̶w̶.̶n̶e̶x̶u̶s̶m̶o̶d̶s̶.̶c̶o̶m̶/̶f̶a̶l̶l̶o̶u̶t̶4̶/̶m̶o̶d̶s̶/̶3̶3̶9̶4̶6̶"̶]̶x̶S ̶E̶ ̶P̶l̶u̶g̶i̶n̶ ̶P̶r̶e̶l̶o̶a̶d̶e̶r̶[̶/̶u̶r̶l̶]̶
̶D̶r̶a̶g̶ ̶a̶n̶d̶ ̶d̶r̶o̶p̶ ̶t̶h̶e̶ ̶c̶o̶n̶t̶e̶n̶t̶s̶ ̶o̶f̶ ̶t̶h̶e̶ ̶d̶o̶w̶n̶l̶o̶a̶d̶e̶d̶ ̶z̶i̶p̶ ̶f̶o̶l̶d̶e̶r̶ ̶i̶n̶t̶o̶ ̶t̶h̶e̶ ̶[̶u̶]̶F̶a̶l̶l̶o̶u̶t̶ ̶4̶[̶/̶u̶]̶ ̶g̶a̶m̶e̶ ̶f̶o̶l̶d̶e̶r̶:̶
̶:̶\̶G̶O̶G̶\̶F̶a̶l̶l̶o̶u̶t̶ ̶4̶ ̶G̶O̶T̶Y̶

̶-̶ ̶M̶a̶n̶u̶a̶l̶l̶y̶ ̶d̶o̶w̶n̶l̶o̶a̶d̶ ̶[̶u̶r̶l̶=̶"̶h̶t̶t̶p̶s̶:̶/̶/̶w̶w̶w̶.̶n̶e̶x̶u̶s̶m̶o̶d̶s̶.̶c̶o̶m̶/̶f̶a̶l̶l̶o̶u̶t̶4̶/̶m̶o̶d̶s̶/̶4̶7̶3̶5̶9̶"̶]̶B̶u ̶f̶f̶o̶u̶t̶ ̶4̶[̶/̶u̶r̶l̶]̶
̶D̶r̶a̶g̶ ̶a̶n̶d̶ ̶d̶r̶o̶p̶ ̶t̶h̶e̶ ̶c̶o̶n̶t̶e̶n̶t̶s̶ ̶o̶f̶ ̶t̶h̶e̶ ̶d̶o̶w̶n̶l̶o̶a̶d̶e̶d̶ ̶z̶i̶p̶ ̶f̶o̶l̶d̶e̶r̶ ̶i̶n̶t̶o̶ ̶t̶h̶e̶ ̶[̶u̶]̶F̶a̶l̶l̶o̶u̶t̶ ̶4̶[̶/̶u̶]̶ ̶d̶a̶t̶a̶ ̶f̶o̶l̶d̶e̶r̶:̶
̶:̶\̶G̶O̶G̶\̶F̶a̶l̶l̶o̶u̶t̶ ̶4̶ ̶G̶O̶T̶Y̶\̶D̶a̶t̶a̶

Starting and running Fallout: London

1. Navigate to the folder you have Fallout 4 installed in and double-click f4se_loader.exe: https://i.imgur.com/H083m8u.jpeg

You may of course configure and reconfigure the graphical settings to your liking resp. what your machine allows for at any time via Fallout4Launcher.exe located in the Fallout 4 folder, just make sure you don't click "Play": https://i.imgur.com/DN2eqIN.jpeg
Always start and run Fallout: London by double-clicking f4se_loader.exe in the Fallout 4 folder.

And that's it - I'm currently at 12 hours into Fallout: London and only had 2 or 3 crashes.

Downloading and applying current and future updates

1. In your library bring up the entry for Fallout: London: https://i.imgur.com/XSHdnTz.jpeg

2. Download the offline installer for the patch/hotfix/update, in this case patch_fallout_london_2.31_Revision2_(75054)_to_V1.01_IV2.31R3_(75422).exe, for example, to the same folder where the offline installers for the base game are: https://i.imgur.com/RLC9HTC.jpeg

3. Again, make sure that the file(s) have been downloaded correctly (check the file size(s)) and, in case of multiple patch offline installers (1x .exe and several .bin files) that they are located in the same folder

4. Navigate to the above mentioned folder containing the patch offline installer(s) donwloaded earlier

5. Double-click patch_fallout_london_2.31_Revision2_(75054)_to_V1.01_IV2.31R3_(75422).exe (future patch offline installers will be named differently of course), and follow the install instructions

6. Once it's done installing, close the patch installation wizard

7. Navigate to the folder you have Fallout: London installed in and double-click installer.exe: https://i.imgur.com/gEqJrgq.jpeg

8. Click on Update and for the patched/updated version to be re-applied to the Fallout 4 installation, on Install: https://i.imgur.com/THOkmRq.jpeg
https://i.imgur.com/dpK2PL4.jpeg

9. Once it's done installing/copying/re-integrating, close the installation wizard

10. From the official patch notes: Navigate to your Fallout 4 Data folder: \GOG\Fallout 4 GOTY\Data and double-click on LondonWorldSpace - ClearUp.bat (this .bat is a simple script that will automatically delete any files that aren't needed anymore, this is required)

11. Navigate to the folder you have Fallout 4 installed in and double-click f4se_loader.exe: https://i.imgur.com/H083m8u.jpeg

12. Once a save is loaded up, an in-game text message ("Fallout: London Updated to v1.01") should appear, further confirming that the patch/hotfix/update has been applied successfully: https://i.imgur.com/NbqQ1Vk.jpeg

Since all the settings (except for the key bindings) have been reset you will have to reconfigure the graphical settings to your liking resp. what your machine allows for via Fallout4Launcher.exe located in the Fallout 4 folder, just, again, make sure you don't click "Play": https://i.imgur.com/DN2eqIN.jpeg
and start and run Fallout: London by double-clicking f4se_loader.exe in the Fallout 4 folder.

JomVanFliet: 5. Double-click patch_fallout_london_2.31_Revision2_(75054)_to_V1.01_IV2.31R3_(75422).exe (future patch offline installers will be named differently of course), and follow the install instructions

6. Once it's done installing, close the patch installation wizard

itsaMeStorm: It said "Some errors occurred, see patch_log.txt for details". Patch log said that Fallout 4 files not found on my system drive, but I have Fallout 4 and London installed on my gaming disk :( There is no freaking way I'm gonna through that download-downgrade-download-mod(s) installation hell again...

I tried to update London installation with installer.exe over existing Fallout 4 + London installation, still doesn't work

seskzklk: Like it states at the very beginning of the tutorial in the OP:
For this manual way of installing and setting it all up, without Galaxy and using only the offline installers it's crucial to start from a blank slate with no remnants of previous installations of Fallout 4 or Fallout: London left behind on your system.
If something goes awry and not according to plan during any of the steps you will have to start from the beginning again.

seskzklk: If something goes awry and not according to plan during any of the steps you will have to start from the beginning again.

Wodenke: All it needs is a little patience and discipline to meticulously follow the OP's instructions.

JomVanFliet: Requires that you own the GOG version of Fallout 4:
https://www.gog.com/game/fallout_4_game_of_the_year_edition

and that we're going to do a clean, from scratch and completely untouched installation of both Fallout 4 and Fallout: London. Meaning you'll have to make sure that there are no remnants of previous installation attempts still present on your machine.

We also won't be using the optional Galaxy client at any point, so if you have it up and running, shut it down.

Downloading the offline installers

1. While logged into the GOG website (https://www.gog.com) go into your library (https://www.gog.com/account)

2. Bring up the entry for Fallout 4: https://i.imgur.com/clarF4Z.jpeg

3. Click on it to expand: https://i.imgur.com/iTZOn9L.jpeg

4. Ignore the big blue "Download and Install Now", click on "Download Offline Backup Game Installers" instead to expand: https://i.imgur.com/mcLVsIq.jpeg

5. Ignore the installers for the "High Resolution Texture Pack" a little further down and download all 10 files ("Fallout 4: Game of the Year Edition (Part 1 of 10)" to "Fallout 4: Game of the Year Edition (Part 10 of 10)") to a folder somewhere on your drive: https://i.imgur.com/fSLigWo.jpeg (may have to change a certain visibility setting to have the .bin files show)

6. Make sure that all files have been downloaded correctly (check the file sizes) and are located in the same folder

7. Repeat the process for Fallout: London:
https://i.imgur.com/m35gCWh.jpeg
https://i.imgur.com/bzBeW5F.jpeg
https://i.imgur.com/iMOZGHx.jpeg

Installing Fallout 4 & Fallout: London

1. Navigate to the folder containing the 10 files for Fallout 4 (1x .exe & 9x .bin files) downloaded earlier - again make sure that all of them are present in the same folder and have been downloaded correctly

2. Double-click setup_fallout_4_game_of_the_year_edition_1.10.163.0_(64bit)_(66700).exe, and follow the install instructions

3. Once it's done installing, close the installation wizard

4. Repeat the the process for Fallout: London

Integrate Fallout: London into/Let Fallout: London take over Fallout 4

1. Navigate to the folder you have Fallout: London installed in and double-click installer.exe: https://i.imgur.com/gEqJrgq.jpeg

2. Follow the install instructions (it should automatically detect the location where you have Fallout 4 installed in, double-check if it's the correct path under "Fallout: London files will be installed in:", click "Change location", if necessary)

3. Once it's done installing/copying/integrating, close the installation wizard

Redundant since Buffout 4 already is and will be included with Fallout: London from v1.01 and onwards [̶b̶]̶I̶n̶s̶t̶a̶l̶l̶i̶n̶g̶ ̶B̶u̶f̶f̶o̶u̶t̶ ̶4̶ ̶&̶ ̶x̶S̶E̶ ̶P̶l̶u̶g̶i̶n̶ ̶P̶r̶e̶l̶o̶a̶d̶e̶r̶[̶/̶b̶]̶

̶A̶s̶ ̶d̶e̶s̶c̶r̶i̶b̶e̶d̶ ̶[̶u̶r̶l̶=̶"̶h̶t̶t̶p̶s̶:̶/̶/̶w̶w̶w̶.̶g̶o̶g̶.̶c̶o̶m̶/̶f̶o̶r̶u̶m̶/̶f̶a̶l̶l̶o̶u̶t̶_̶l̶o̶n̶d̶o̶n̶/̶f̶a̶l̶l̶o̶u̶t̶_ ̶l̶o̶n̶d̶o̶n̶_̶q̶u̶i̶c̶k̶_̶s̶t̶a̶r̶t̶_̶g̶u̶i̶d̶e̶_̶c̶o̶m̶m̶o̶n̶_̶b̶u̶g̶_̶f̶i̶x̶e̶s̶/̶p̶o̶s̶t̶1̶"̶]̶h̶e̶r̶e̶[̶/ ̶u̶r̶l̶]̶:̶

̶-̶ ̶M̶a̶n̶u̶a̶l̶l̶y̶ ̶d̶o̶w̶n̶l̶o̶a̶d̶ ̶[̶u̶r̶l̶=̶"̶h̶t̶t̶p̶s̶:̶/̶/̶w̶w̶w̶.̶n̶e̶x̶u̶s̶m̶o̶d̶s̶.̶c̶o̶m̶/̶f̶a̶l̶l̶o̶u̶t̶4̶/̶m̶o̶d̶s̶/̶3̶3̶9̶4̶6̶"̶]̶x̶S ̶E̶ ̶P̶l̶u̶g̶i̶n̶ ̶P̶r̶e̶l̶o̶a̶d̶e̶r̶[̶/̶u̶r̶l̶]̶
̶D̶r̶a̶g̶ ̶a̶n̶d̶ ̶d̶r̶o̶p̶ ̶t̶h̶e̶ ̶c̶o̶n̶t̶e̶n̶t̶s̶ ̶o̶f̶ ̶t̶h̶e̶ ̶d̶o̶w̶n̶l̶o̶a̶d̶e̶d̶ ̶z̶i̶p̶ ̶f̶o̶l̶d̶e̶r̶ ̶i̶n̶t̶o̶ ̶t̶h̶e̶ ̶[̶u̶]̶F̶a̶l̶l̶o̶u̶t̶ ̶4̶[̶/̶u̶]̶ ̶g̶a̶m̶e̶ ̶f̶o̶l̶d̶e̶r̶:̶
̶:̶\̶G̶O̶G̶\̶F̶a̶l̶l̶o̶u̶t̶ ̶4̶ ̶G̶O̶T̶Y̶

̶-̶ ̶M̶a̶n̶u̶a̶l̶l̶y̶ ̶d̶o̶w̶n̶l̶o̶a̶d̶ ̶[̶u̶r̶l̶=̶"̶h̶t̶t̶p̶s̶:̶/̶/̶w̶w̶w̶.̶n̶e̶x̶u̶s̶m̶o̶d̶s̶.̶c̶o̶m̶/̶f̶a̶l̶l̶o̶u̶t̶4̶/̶m̶o̶d̶s̶/̶4̶7̶3̶5̶9̶"̶]̶B̶u ̶f̶f̶o̶u̶t̶ ̶4̶[̶/̶u̶r̶l̶]̶
̶D̶r̶a̶g̶ ̶a̶n̶d̶ ̶d̶r̶o̶p̶ ̶t̶h̶e̶ ̶c̶o̶n̶t̶e̶n̶t̶s̶ ̶o̶f̶ ̶t̶h̶e̶ ̶d̶o̶w̶n̶l̶o̶a̶d̶e̶d̶ ̶z̶i̶p̶ ̶f̶o̶l̶d̶e̶r̶ ̶i̶n̶t̶o̶ ̶t̶h̶e̶ ̶[̶u̶]̶F̶a̶l̶l̶o̶u̶t̶ ̶4̶[̶/̶u̶]̶ ̶d̶a̶t̶a̶ ̶f̶o̶l̶d̶e̶r̶:̶
̶:̶\̶G̶O̶G̶\̶F̶a̶l̶l̶o̶u̶t̶ ̶4̶ ̶G̶O̶T̶Y̶\̶D̶a̶t̶a̶

Starting and running Fallout: London

1. Navigate to the folder you have Fallout 4 installed in and double-click f4se_loader.exe: https://i.imgur.com/H083m8u.jpeg

You may of course configure and reconfigure the graphical settings to your liking resp. what your machine allows for at any time via Fallout4Launcher.exe located in the Fallout 4 folder, just make sure you don't click "Play": https://i.imgur.com/DN2eqIN.jpeg
Always start and run Fallout: London by double-clicking f4se_loader.exe in the Fallout 4 folder.

And that's it - I'm currently at 12 hours into Fallout: London and only had 2 or 3 crashes.

Downloading and applying current and future updates

1. In your library bring up the entry for Fallout: London: https://i.imgur.com/XSHdnTz.jpeg

2. Download the offline installer for the patch/hotfix/update, in this case patch_fallout_london_2.31_Revision2_(75054)_to_V1.01_IV2.31R3_(75422).exe, for example, to the same folder where the offline installers for the base game are: https://i.imgur.com/RLC9HTC.jpeg

3. Again, make sure that the file(s) have been downloaded correctly (check the file size(s)) and, in case of multiple patch offline installers (1x .exe and several .bin files) that they are located in the same folder

4. Navigate to the above mentioned folder containing the patch offline installer(s) donwloaded earlier

5. Double-click patch_fallout_london_2.31_Revision2_(75054)_to_V1.01_IV2.31R3_(75422).exe (future patch offline installers will be named differently of course), and follow the install instructions

6. Once it's done installing, close the patch installation wizard

7. Navigate to the folder you have Fallout: London installed in and double-click installer.exe: https://i.imgur.com/gEqJrgq.jpeg

8. Click on Update and for the patched/updated version to be re-applied to the Fallout 4 installation, on Install: https://i.imgur.com/THOkmRq.jpeg
https://i.imgur.com/dpK2PL4.jpeg

9. Once it's done installing/copying/re-integrating, close the installation wizard

10. From the official patch notes: Navigate to your Fallout 4 Data folder: \GOG\Fallout 4 GOTY\Data and double-click on LondonWorldSpace - ClearUp.bat (this .bat is a simple script that will automatically delete any files that aren't needed anymore, this is required)

11. Navigate to the folder you have Fallout 4 installed in and double-click f4se_loader.exe: https://i.imgur.com/H083m8u.jpeg

12. Once a save is loaded up, an in-game text message ("Fallout: London Updated to v1.01") should appear, further confirming that the patch/hotfix/update has been applied successfully: https://i.imgur.com/NbqQ1Vk.jpeg

Since all the settings (except for the key bindings) have been reset you will have to reconfigure the graphical settings to your liking resp. what your machine allows for via Fallout4Launcher.exe located in the Fallout 4 folder, just, again, make sure you don't click "Play": https://i.imgur.com/DN2eqIN.jpeg
and start and run Fallout: London by double-clicking f4se_loader.exe in the Fallout 4 folder.