If someone is living abroad they are generally not paying NI or registered with a GP or have a perm address in the UK.
GP's don't unregister people very quickly - in fact, I went over 4 years without seeing a GP and was still registered on their books. In fact, they didn't even ask about me in that time - and they were still perfectly happy to give me an appointment when I needed one.
As I said - you can not pay NI or be registered with a GP and still be perfectly happily living in the UK. There is no effective way to prove whether or not a British citizen is entitled to use the NHS. It's not like Poland.
As for not knowing who is entering and leaving, you really think the government dont know? Dont worry when the ID cards becomes compulsary, the UK will have exact records of who is entitled and who isnt ;0) Roll on that day!
Yes, I think they don't know. For a start, there's quite a large unguarded border with the Republic of Ireland in which no controls whatever are used. The Irish authorities certainly don't share information with the British!
As for the comments about ID cards - not withstanding the fact that there are huge civil liberty issues, the Scottish Government has already categorically said that they will NOT be using them for anything government-related - which includes public health care, among lots of others. There's next to no chance of Labour winning a majority in the next election - and the Tories and Lib Dems are against ID cards. The system is very, very likely to be scrapped, not least because the next government is going to have to cut back public services dramatically.
The NHS ultimately works on an honesty system - and if someone has paid a significant amount in the UK system and had little in return, what motivation is there for honesty?
Lir - the UK has no concept of permanent residence of a person, except for tax purposes - which is incredibly complex and subject to so many rules that it's impossible to give blanket advice. Yes, in theory, people are no longer entitled - but there is no way of checking eligibility conclusively.
What do you honestly think the NHS would do with someone who turned up at a doctor, who said that they had been homeless for a while and who was clearly a Brit? They'd treat him, of course. He might be asked to declare that he was in the UK - but this is entirely up to his/her own honesty and nothing else.
I think the vast majority of British citizens aren't too bothered if their fellow citizens use the NHS anyway, even if they're not entitled to in theory.