Fallout: London is a huge Fallout 4 mod that is now playable—and worth playing
It took a crew of more than 100 talented modders and another hundred voice actors nearly five years to make Fallout: London. Just as they planned to release it, Bethesda came out with a "next-gen upgrade" of the mod's base game, Fallout 4, forcing the team to scramble and ultimately find a way to downgrade the game. When they finally released London, they then dealt with game-stopping bugs and quality issues that their small QA team could not have caught. It's been a long, maybe even post-apocalyptic road for these modders.
A few updates later, Fallout: London is in much better shape. I've been able to put about 12 hours into it, and that, in itself, is essentially my review: it is worth that kind of time and more. If you can still enjoy Fallout 4, of course.
Any Fallout fan waits a long time between official releases, so it can be tempting to go easy on any new offering, however spit-and-bailing-wire it may seem. But Fallout: London is a game in its own right, with a distinct look, vision, and stories to tell. You can find evidence of its unofficial mod-ness if you look around, but you're better off doing the Fallout thing: wandering, wondering, fighting, and occasionally talking to some messed-up weirdo.