If you have continuously been registered with a GP using a U.K. address then should be no problem,but say you’ve been out of U.K. and not registered for many years red flags might occur. A hospital visit will entail them wanting to know who your GP is. There is also 'connectivity' whereby government departments share information. So say you renewed your U.K. passport in Thailand or tax authorities / DWP (pension) have you living in Thailand then you could get found out.