It's for security purposes for the Olympics that will be held in London. This has got to be one of the stupidest ideas I've ever heard. What do they think will happen if they fire one of these things? Here's the coverage from The Guardian: London rooftops to carry missiles during Olympic Games and for a little analysis Olympics 2012: A Bruce Schneier Moment
I am reasonably certain that their hopes are (if one is fired off), that it will hit the incoming aircraft they have deemed a threat. I am not really certain on the thinking behind it, and the planning they have placed into it, but that is the militaristic/defensive posture thought pattern I have gleaned from it. While this may be a bit of overkill from an outside looking in POV, perhaps they have some intel or other reason for doing this at this time.
bolding mine. This is something out of the Darwin awards. Most of the civilian casualties in Bhagdad came from Sadam's own AA fire. If fired, these things will come down in some London neighborhood. I am reasonably certain the US put them up to it.