Fallout: London is a massive fan-made Fallout 4 mod promising a "DLC-sized" adventure through post-apocalyptic Europe.
The Fallout: London mod team describes the project as "a continuation of the base game" set in an entirely new environment with its own unique culture. That said, it might be easier to visualize this mod as an extension of Fallout 4 rather than a direct continuation; it's set in the year 2237, putting it roughly 50 years ahead of the events of Fallout 4 and smack in-between Fallout and Fallout 2.
Over on the Fallout: London Discord server, the mod team explained that the mod's map is "about the size of the vanilla Fallout 4 Commonwealth" and, in London's geography, extends from Westminster to the edge of Bromley, which is a pretty sizable chunk of the city. Inside, you'll find new "factions, storylines, NPCs, weapons, creatures" and more. One of these new factions seems to be a gaggle of modern-day knights in full plated armor swinging claymores like some kind of medieval Brotherhood of Steel.
Just as importantly, you won't find several Fallout staples in this mod since London is new territory for the series and therefore devoid of American-made groups and anomalies, from the Brotherhood of Steel to Super Mutants.
"Vault-Tec is entirely an American company, thus it will have no presence in London," the mod's FAQ reads. "We will have a version of a vault, but it will be entirely different from that of Vault-Tec, except that it is a shelter, beneath the ground. So don’t expect Better Living, underground. In a similar vein, there will likely be a British counterpart of Vault Boy, in terms of a popular mascot that will make frequent appearances. Certain robots such as Mr. Handys and Protectrons had gone global by 2077 and thus will be included."
Fallout: London is such a bottom-up overhaul that you'll need a new Fallout 4 save to play it (not to mention all the DLCs, which are thankfully pretty cheap at this point), as the mod "will make critical changes to the base game's mechanics" as well as its content, the mod team says. For example, while the mod won't come with voice lines for your character, the team is looking to expand the game's dialogue system to bring it more in line with previous Fallout games.
Fallout: London doesn't have a release date just yet, in large part because it's being made by a team of volunteers. Unsurprisingly, the mod is only planned for PC. If you're hungry for updates, you can follow the project on Twitter.
The next official Fallout update is Fallout 76's Steel Reign, which will conclude the Brotherhood of Steel arc on July 7.