If you have no other choice but to go nuclear then can I suggest a single 20 Mega-Tonne in the centre of London should be enough. I'm sure you've already worked it out (there's plenty of websites where you can choose a city and the yield and it'll show you the outcome) and 20 MT should wipe out most of our career politicians, their bullshit friends, and all of the crap that lives within M25. The good people of the UK of GB that are left would be most grateful to you, we'll round up the rest of our shit politicians and stick them against the Proverbial-Wall. Our lives would be a lot better without them and that Shit-Hole-London keeping us back, enabling the rest of the UK of GB to get on with our lives and prosper.

If you could also wait until the wind is blowing over to the south-east then that would be even better!