So my Fallout 4 on Steam won't start unless I lobotomise my game with the 'next gen' update - with all of its bugs and lack of bug-fixes, and I think those of use who want to keep our game working, should have the ability to swap for a version that won't force this crap on us.

No, GOG doesn't offer a trade in program for steam to gog copies.

Sorry to be the bearer of bad news,
-Me

not any more anyways, they used to have it in the past though, got a few GOG games that way.

They got rid of that some time ago, because it consisted of them footing the bill, much like Epic.

since London is newly released and will have problems for a long time, like most new released do, i would do 1 of 2 things.

Its free, so just keep it around maybe one day FO4 GOTY on GOG will drop to a reasonable price (currently high price due to London) and you can buy it again on GOG if you want to mess with London. That way you can play London and try it out on GOG, and keep your Steam version for non London.

Plus later by that time hopefully many bugs and problems will be fixed, maybe even the steam update that has messed everything up will be fixed and London will work on the Steam version without jumping through hoops. There are thousands of other games to play, Shelf London until one of those 2 things happen. Personally i wouldn't rebuy FO4 on GOG just for that. So wait until London or Steams update is more stable and fixed so both will work fine is better idea.

Steam is the worst. I avoid it like the plague, and only buy games on GOG and Epic.