nigelh
I would have thought there would be so many places they lose contact if you are v low that it would be impossible to prove the contact was you ?
Have you thought about when you are flying close to the radar heads? The modern ones have a very good 'look down' capability and can see you down to ground level. Do you know where the radar heads are, so that you can avoid them? Did you know that, in most modern countries, their pictures are all linked into a central system and recorded?
Have you considered the many AEW, AWACS and ASTOR aircraft that fly around our airspace and can even watch road vehicles travelling along the ground? Most of their radar pictures are downloaded into the Air Defence Radar systems and are also recorded. It then becomes very easy to track your route, your origin, your destination and flight times even if you have your transponder turned off!
If you want to break the rules, that's up to you. But don't expect that you are not going to get caught.