If you want to play that whopping Fallout London mod you'll need to downgrade Fallout 4's "next gen" version

rockpapershotgun.com:

We haven't written nearly enough about Fallout London, a gargantuan Fallout 4 total conversion that takes place in a whole new map based on England's capital city - the Big Smoke and/or Great Wen, whose real-life incarnation probably harbours at least one nuke at any given moment, though I haven't asked the King about that lately. Best not speculate.

Anyway, Fallout London is a huge effort from modders Team Folon that was recently sabotaged when Bethesda treated Fallout 4 to a next gen update to capitalise on the Amazon TV show's popularity, rendering several mods incompatible and breaking a lot of stuff. Team Folon have now fiddled with the workings and sent a Fallout London build to GOG for QA testing, which suggests that release is imminent. The catch, however, is that you will need to downgrade your copy of Fallout 4 to play the mod, because even with Team Folon's last-minute adjustments, Fallout 4 next gen and Fallout London simply don't get on.

As such, Fallout London will ship with its very own installer and downgrader. "GOG have been amazing through all of this," project lead Dean 'Prilladog' Carter explained on the game's Discord this week, via VG247. "To be honest, we've been the ones causing them the issue. What with us having things break due to the next-gen [update] and then needing the 3rd parties to update, then waiting for them to be fixed, only for the fact that at the 11th hour we've discovered that the next-gen [version of Fallout 4], even after updates, isn't stable enough and thus we are now going out on the old version - [hence] the need for a downgrader."

While I think the scale of the backlash to Fallout 4's "next gen" revamp was perhaps slightly exorbitant, as is often the case with backlashes, Fallout London seems worth the downgrade. It's said to be around the size of the main game's Commonwealth map and Far Harbor DLC combined, and spans the capital’s boroughs from Camden to Croydon, Westminster to Hackney.

You'll get to fight