7.62....... I think not
The big problem with a shoot down over the London area is what happens with the rounds that miss the target. Just think what 20 or so 7.62 rounds would do if they went down An East London high street ?
You could end up with 10-15 people killed just because a student pilot had got lost, I would think that a far better way of doing business would be one 0.5 round fired into the engine of a light aircraft.
The result would be the instant stoppage of the engine and a good chance that the aircraft would stay controllable enough to make a controled forced landing.
This would minimize the chance of the great unwashed of east London being on the sharp end of live rounds or bits of aircraft falling from the sky.