What about a Fallout RP set in the ruins of London? Some possible factions:

[b]The Monarchy:[/b] Led by the Monarch, a warlord squatting in the ruins of Buckingham Palace, claiming to be descended from the old royal family.
[b]The Whitehall Republic:[/b] The remnant of the former English government and military in Westminster, directly opposed to The Monarchy.
[b]Cathedral City:[/b] A loose community of outlaws, raiders, chem pushers and whores operating out of the ruins of St Paul's Cathedral.
[b]The Ruskies:[/b] Descendants of the Communist Soviet forces stranded on English soil after the Great War, operating out of a derelict Soviet aircraft carrier beached at the mouth of the Thames.

Another possible hazard in the London Wasteland would be the London Underground. Full of all manner of ghouls and mutant beasties. Not to mention a ruthless slaver ring operating out of King's Cross Station.

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Huh. That ended up a lot more fleshed out than the basic idea I started with.