Would setting up class D airspace for 50nm around London, and keeping a well armed helicoptor hovering 24/7 over any stadium in use not be an ecomonically cheaper solution?
Someone turning off their cleared track, and making a bee-line for the statium could easily be shot down.
It would be a lot more effective than trying to scramble aircraft to intercept when someone who has filed their flight plan and got their clearance number suddenly leaves their cleared track!