robvee. It has nothing to do with citizenship. If you are a legal permanent resident in another country then legally you are no longer eligible for NHS care. Things have changed considerably in recent years. Yes there are some ways round this, but they are technically not legal.
"Under the current Department of Health Regulations, anyone spending more than three months outside the UK is no longer automatically entitled to free NHS treatment," confirms Katrina Osman, broker with IHC International. "And, therefore, NHS charges may apply for any hospital treatment received. Those in receipt of UK state pensions, and who have previously lived in the UK for a continuous period of 10 years, remain entitled to some cover, although this is dependent on their current country of residence and length of time there." NHS Rights for Expats