I suspect it is a show of force and the missiles on top of tower blocks are actually empty tubes.
Otherwise, if one had terrorist tendancies, wouldn't it be easier to enter said tower block with a team, go to the roof and shoot the squaddies who're probably dozing in the sun, then nick their missiles and shoot down an airliner yourself?!
I am sure MI6 have thought of this which is why I bet the tubes are empty
