The thing about the UK, which is almost unique ffrom anywhere else in the world, is, that we're a small and mostly highly populated island.
Accordingly, even if you are in an area of "open countryside", you're never very far away from either a major airport or an area/airfield where G/A operates - be it a microlight or herd of PA28s. The locals get used to what goes on, how, and at what times - anything outside these parameters, and they get suspicious and will tell someone, and this has been proven on many occasssions.
I accept that if someone is fanatical enough to want to cause a major incident, then it's not always possible to stop them, however, the current procedures and present vigilance of curiosity which we have in the UK is more than adequate to supplement the official laid down requirements.