Fallout: London is the newest mod for Fallout 4, and players will want to know how to install and play the game. You should be aware that it will only work on PC versions of the game, and that it will irrevocably change your Fallout 4 save. That makes it important to plan accordingly, and the installation itself can be a bit tricky depending on what version of the game you have.
Fallout: London is a brand new mod for Fallout 4, which at last allows players to travel to the British wasteland and head overseas. After spending the vast majority of time in the American wasteland, it’s a nice breath of fresh irradiated air. Players will have a blast with this mod featuring new characters, a new story, and new weapons if they can install it.

Fallout: London Mod Boasts Doctor Who Voice Talent
Fallout: London features Doctor Who stars Colin Baker and Sylvester McCoy, who play scientists that introduce players to the post-nuclear Big Smoke.
Installing Fallout: London Through GOG
The Easy Way
The GOG Galaxy 2.0 store is going to be the easiest way to purchase, , and play Fallout: London. If you do not already own Fallout 4 or by some chance you bought it directly from GOG, you should count yourself lucky.
Go through the storefront to purchase the Game Of The Year Edition of Fallout 4 - you must have every DLC to play Fallout: London and this is the easiest way to get it. Otherwise, you will have to purchase whatever DLC you don't own.
Fallout: London is not available on the Epic Games Store version of the game.
You then must disable cloud saves, Fallout: London directly from the GOG store for free, and then launch it. Following the launcher’s instructions will get you ready to play the new mod through the quickest and most direct route possible.
Installing Fallout: London On Steam
Downgrading
Unfortunately, with the update to current-gen standards, Fallout 4 has made a lot of mods unplayable on the newest version of the game. Therefore, players who own Fallout 4 and want to play Fallout: London through Steam will have to downgrade their version of the game. The version you are looking for is 1.10.163.0. To do so, YouTuber John Doe has a great video explanation of completing the downgrade manually, but there is another option.
Instead of having to do so yourself, you can instead use this Downgrader mod from Nexus Mods, and it will make the whole process simpler. It is still a fairly lengthy process, so if you have the means to and buy it from the GOG store, that is the recommendation. You will also never have to worry about Steam deciding to update your game at random.
Once you have made the downgrade, you still must get Fallout: London from the GOG store and run the installation launcher. You will immediately be able to it to your Steam file, just be sure that the launcher has detected the Steam file for the program location. And then you will be all set and ready to go to play Fallout: London.
Video Credit: YouTube|John Doe