I haven’t installed it or tried it out myself but it is being hosted here on GOG which is neat. Here is an article a bit about why GOG is hosting it.
The Steam version apparently requires the use of this downgrader mod from the Nexus.
You will need the GOTY edition with all the DLC either way.
If you want to see how the mod is several people are already streaming it.
Here is some information from the GOG page:
Summary
Fallout: London lets you explore a dishevelled London set in the Fallout universe. It overhauls Fallout 4 with a new story, new weapons, London-centric items and a vast recreation of the ruined city to explore.
Fallout: London is a free total conversion mod for Fallout 4, available to anyone who owns the base game and all of its DLCs. It is developed non-commercially by Team FOLON, many of whom are proud London residents or expats.
About This Game
There has never been a better time to sample the capital’s delights! Fallout: London lets you explore the wonders of the city 160 years after the bombs have dropped, and 40 years before the events of Fallout 3. You can now reimagine Fallout in a London-centric universe, interacting with a brave new world of London-based locations, stories, factions, creatures, weapons, and gameplay mechanics.
Features
Protect and Survive!
The Gentry-dominated London stands on a knife-edge, and you may be the key to which way the guillotine falls. Play a new story set in London with a fresh character beginning, numerous conflicting factions to engage with and multiple story paths and endings with up to 90 hours of gameplay. Just be aware of what terrible secrets you may uncover in the shadowy gaps amidst the halls of power.
- 200 Quests to change London’s future
- 20 Factions bickering for power
- 7 Companions to bring along on your journey
- 15 Boroughs crafted from the ground up, bringing you a new warped and broken London
- Extended Dialogue System to bring player-choices to the table, carve your own story!
- Overhauled Original Soundtrack to bring personality to each nook and cranny
- Original Voice Acting from all major characters, gangs, and factions
Authentic London Flair!
Discover new weapons, creatures, clothing, items, collectables, architecture, foliage and NPCs to complete the London setting. Interact through the world with your custom Atta-boy, with a fresh interface based on the UK’s life-saving Protect and Survive nuclear information campaign.
- Build back better with 7 unique settlements
- New craftable items with a plethora of new weapons
- Dynamic new animations to bring life
Now I have to do a clean install and downgrade again. Too many mods installed already
You could just select a new install location for the game and keep that modded setup as well.
I just bought FO4 again on GOG and that made it a completely pain-free install of like 6 clicks, no downgrade needed, 0 issues except engine-inherent thing like the occasional CTD or dark face bug.
I just considered it as paying the $15.99 for London, tbh.