so right now, theirs a lot of weird ways its not working for me, I neglected to write down exact cause and effect, but it boils down to these:

in gaming mode, weather I use lutris or run it directly through the f4se launcher, it will either crash, or give an error message telling me to enable loose files, I've check, and archive invalidation is there, I don't understand whats going wrong there.

I get the same loose files issue in desktop mode in lutris through gog galaxy, but if I run the default launcher first and then run london the game will act correctly, the problem is that there is no way to interact with the game, its just stuck on the main menu.

I would love to get this running on the deck, so if someone has suggestions/solutions/explanations that would be awesome.

EDIT: I believe the issue is related to the flatpak system, using flatseal to give lutris full access to files seems to have solved the issue. it works pretty well now, however, it seems that the game is fairly demanding to the deck and I'm scared of melting my decks hardware. so meh.

While not directly on deck, but I play it on linux (where is also lutris, heroic or wine, or even Steam proton... ) and the game runs just fine... although I had to heed a lot of installation details, that are out of scope of linux, as FO London is a bit picky on how you install it.
I also added extra mini-mods (but that was only to improve performance, FPS and lower the drop-to-OS ratio)
Said that, if installed properly, also on deck it should work.

I played it for a week on the Steam Deck. I tried installing it using Lutris both offline & GOG Galaxy installs, and I kept getting issues. Some major, some minor, but in the end it was too much a pain in the butt to deal with so I went to the downgraded Steam version route.

There is a YouTuber (Deck Wizard is his channel) that has a solid tutorial HOWEVER the version of the downgrader tool he used is the "1.5.1" version, and unless you like compiling source code only use that one (the only Linux pre-compiled version out there). Follow the instructions to a T. However again he has a "performance boost" video while yes do install the recommended files IN NO WAY should you alter the one file he tells you to alter (I will explain later).

Now some thing you will need to know if you go this route unless devs release patches in ZIP/RAR files, or add a option to just extract them for manual installs then forget about updates. the GOG installers will give you a error due to it being made for Windows. You can not run it in desktop mode more on that later too. Side effects are weird & wonderful at the same time, but crashes are fewer than running the non patched version on Windows. Some side effects are you get are like the following... Green text like the official FO4 instead of white, you can still get some Steam achievements even though your running a mod (I gained 4), 16GB of total of RAM (both for system & GPU) is NOT enough (more on that later), and some loading of assets can be slow even on a SSD (DO NOT INSTALL on Micro SD card).

The biggest issue on having it on the Steam Deck is the 16GB of total system RAM even at 720p or 800p resolution this mod will require more than it has. In the tutorial Deck Wizard wants you to turn off Memory Management while it's fine on a Desktop with 16GB of RAM & 2-8GB of VRAM the Steam Deck will fight over that precious 16 causing massive slow downs in certain areas, and/or during certain weather conditions (12fps to 16fps). In fact once you play with it long enough or go to certain areas it will cause a issue were the Deck is drawing more RAM than you have your FPS will drop to 1fps, the controls will become unresponsive, and you will be holding the power button to do a emergency shutdown. The second time actually caused a issue with the Steam Deck UI so I had to do some clean up work to fix the issue. If you run it with Memory Management turn on it's fine. Yes you will see the fps drop below 30, you may have a stutter or two in intense areas, but it won't go to 1 with you have to fix your OS in the process.

I've been stuck in a hospital for a week with my Steam Deck enjoying my time in London did 24+ hours with no major hiccups after turning back on Memory Management. Certain areas like The British Museum, The Sewer Market, and other little areas will eat your RAM up, and you will dip as low as 14fps in these areas, but the rest will be from 60fps to 30fps with not many problems. It's way better than the Lutris route IMHO. This is the best way to go for the Steam Deck, or any handheld with only 16GB total RAM.

I tried to post a picture when it reached 19GB of RAM with 2GB being virtual memory (that is when the PC uses your storage as RAM) , but for some reason it failed to show it. So F it I ain't going to try a third time. :P