I don't get it anyway. In order to register with a GP, you need to prove that you live in the catchment area. So how much harder is it to prove you have a right to use the NHS.
I was here in 06 and needed to see a GP. I went in (granted I told them I wasn't eligible, but the took all my documents and double checked) and then charged me. Most GPs already charge for things like vaccinations for holidays, so I am sure they can just charge for this as well.