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
I seem to remember an incident in NI circa 61 when a RN officer was convinced we were asleep on watch. A rifle butt across the back of his hand followed by the muzzle up his nose & the obvious sound of the bolt being cycled convinced him otherwise.
However, give it a go you may be right.