To play the Fallout: London mod, you'll need to downgrade Fallout 4
, by Thibaut
Delayed at the last moment because of the next-gen version of Fallout 4, the Fallout: London mod is finally coming out via GOG. However, the devs had to make one major concession in order to release it.
The mod team has stated that the next-gen update of Fallout 4 isn't stable enough to run the game properly. To make things right, the installer will include a software that downgrades the game.
This is a must for Fallout: London to work as intended.
Source: eurogamer.net