Is one being very slightly too silly/cynical to query at whether this is a mite extreme
Your assumption of a full war-footing reaction to a lost microlight is what is silly. You can expect a very graduated response starting with visual identification of the bogey, then encouragement to turn away. It would take a very determined lost PPL to continue boreing on towards the park with an armed helicopter twenty yards from the cockpit and possibly a warning shot across the nose. The aircraft would have to demonstrate a credible and immediate threat to get the kill and for anything eminating from a flying club would most likely be investigated by a support helo with a heavy machine gun not a missile armed Typhoon. I don't think it's going to be like Guy Fawkes' night, despite what the more hysterical believe.