@cman: sorry didn't saw that part in the QA section.
I'm sure they mixed the folder name for the .inis up!
It should be GOG/Games/Fallout London/_Config (or GOG/Fallout London/_Config in my installation)!
Another tipp from my reddit link
If you're booting up the Steam version of the game via our launcher or f4se_loader.exe, and immediately have an infinite black screen, Steam may be trying to do a "pre-cache shader update", which gets stuck on 0% on heavily-modified versions of the game until you restart Steam. This probably won't be happening to many people but if it does restart Steam, go to Steam > Settings > Downloads, scroll to the bottom and turn off "Enable Shader Pre-caching", and restart Steam again.
If that doesn't help, try delete Fallout4 und reinstall wth a more recent downgrader (search on github for Fallout-London/FOLON-FO4Downgrader). You installed Buffout 4 1.28.6 and xSE Plugin Preloader 0.2.5.1 as recommended? Any crash logs in Documents\My Games\Fallout4\F4SE?