The NHS is quite incompetent at charging people, but some trusts (especially in the South London area) are extremely competent at notifying Immigration about any foreign visitors who may be chargeable. This in turn results in problems at Ports of Entry and/or UK Consular Posts abroad if applying for entry/visas in the future.

Officially, one is required to obtain a clearance certificate at discharge which confirms that no payment is outstanding to the NHS. This is then supposed to be turned over to Immigration when departing the country together with an exit interview to confirm that the treatment was only incidental to the visit. In reality, this is very poorly documented and nobody seems to actually follow it - except that any flags in the databases will automatically trigger on future entries.

I've had to navigate this nightmare not once but TWICE on behalf of crewmembers who had been taken to the A&E with minor injuries at different times and whom the NHS reported to UK Immigration as potentially chargeable. The company offered to pay any charges but they didn't know how much the charges would actually be (supposed to be charged on cost recovery basis) so they just didn't actually invoice BUT still notified Immigration that this was a potentially chargeable case. Absolutely perfect bureaucratic catch-22. The first time took almost 3 weeks to track down the paperwork from the NHS and find someone who knew how to waive the charges (whose value was still unknown) and issue a clearance certificate. Fortunately the ALOs at Immigration were able to retroactively sort things out within a couple days once the clearance certificate was issued, but that was 3 weeks having a crewmember grounded and unable to operate flights to the UK as a result.

Oh, and FWIW, the crewmembers were insured under a comprehensive global policy taken out by the company for all traveling employees, so actual payment of the costs was never the issue. It was simply the paperwork that goes with it.