I think it’s anybody who is allowed to be here.
If you’re on a time limited visa, I think there’s a yearly charge for it. Otherwise you get an NHS number and away you go.
https://www.england.nhs.uk/long-read/the-nhs-number/#allocation-of-nhs-numbers
Well still see Fahrenheit used if it’s a hot sunny day. If it’s a cold day we use Celsius.