https://www.bristolpost.co.uk/news/bristol-news/calls-bri-end-harrowing-upfront-2923519
In my hospital when someone comes in for an emergency appointment they fill a triage form outwith their details, when they hand it back in we ask them two questions.
Have you been treated here before ? (it helps if they have notes)
Have you been living in the uk for the last 12 months(cif the answer is no they fill another form out and the overseas team will contact them and they may have to pay.
This isn't life saving but for emergency dental treatment.
This is exactly the same for expats who have been overseas for 12 months or more.
Contrary to popular belief the NHS in England is residency base and not contribution based.