In fact, the website lists seven factions for you to discover - personally I'm looking forward to encountering the Camelot Knights, they sound like a laugh. The mod FAQs explain that you can expect much more than just a new location - Fallout: London will also provide new factions, storylines, NPCs, weapons and creatures. It feels like there are some nods to Britain's WWI and WWII past, with barrage balloons seen in the sky and soldiers camping out in trenches.
I'm also a big fan of the Beefeater who's embraced a more Raider-inspired look.
There are some really nice touches throughout the trailer: you can spot plenty of London buses, murky tube stations and even a classic London pub.