I mean, if London is the only place thats being attacked just evac as many people as possible, then drop like 20 nukes from a plane, that would vaporize the city and everything in it. Problem solved.
London seems to be a popular target for disaster movies. I can think of two zombie flicks (28 Days Later and Shaun of the Dead) off the top of my head that were both set in London.
Oh those were great movies