Unlike gliders, anyone could buy a microlight or aircraft and have it delivered to an airstrip and fly it away
You could do that with a 747. You can pick up one of those for under £1M. It will probably be past its pressurisation cycle limit so it can't go to normal cruise anymore, but hey who is going to check. Have it delivered to say Manston, jump in, call for a VFR departure for a local flight and off you go, flying a 747 single-pilot. Nobody is going to question it. It's just highly illegal and you will be in huge trouble if/when they catch you. Luckily, anybody with the technical knowledge to pull it off will also be smart enough to not try it.