I do live here now, so it is historical. And I was never non-resident in the UK. I was completely open with the dentist and his admin staff, and they were happy to keep me on. It was because I was never a resident elsewhere - always on temporary working visas (that were usually tied into the job/contract I was doing).
You have to be resident somewhere! And the UK was my resident country. I would have been eligible for NO benefits or welfare in the other countries despite paying full tax there, this is because the UK government has reciprocal agreements with various other governments. Ergo, i was still entitled to the benefits of being a British Citizen and resident.
It would be like saying that a gap year student doing 18 months working holiday around Australia was no longer entitled to being registered with his/her family GP and dental practice.
I'm not upset by the comments - I know I'm in the right and have had it confirmed by the NHS dentist, so I don't feel bad in anyway. And because I have regular check-ups I don't then become a drain on the NHS later, with rotten teeth and having to go to A&E!
Prevention is always better than cure.
FWIW i think it is terrible that people in some areas can't get NHS treatment - that is the dentists and the government's fault - not mine!