I'm sure it would be fairly easy to rent a light aircraft out of Wycombe , White Waltham etc fly reasonably low whilst ignoring all ATC and fly into either head on , or into the side of an aircraft on finals. What notice would ATC have to pass on to agencies who inform the airforce /army to shoot said intruder down??. Enough I would have thought if there are nearby resources to do so.
It would be the easiest thing in the world to base a couple of Apaches at Northholt along with an armed Hawk or two I would suspect.
I'm sure someone somewhere has the intelligence and inventiveness to look at scenarios we are putting forward and may have measures already in place that we dont know about.