There does seem to be other similar sounding posts. In my case I cannot continue or load an existing pre-patch save but I can start a new game from scratch. After playing that just fine, if I then try to change character and load one of my orignal saves it crashes again.

My post - https://www.gog.com/forum/fallout_london/folon_will_not_start_after_the_recent_patch_update
There is at least one other similar sounding post I've found. I'd only got to level 5, I may just start again :-(.

Navigate to the Fallout London install directory, not the Fallout 4 GOTY directory. There will be an exe file called installer.exe. Run that. That should bring up the launcher for Fallout London, where you choose update or play. Click update.

Once the update is done, you need to navigate to the Fallout 4 GOTY install directory, then go into the Data subdirectory and run "LondonWorldSpace - ClearUp.bat". That is a batch file that will clean up the installation folder after the update. (Really, it is removing the BakaScrapHeap script memory extender plugin for the Fallout 4 Script Extender, which was causing a lot of crashes).

Should be good to go after doing these steps.