4 Answers

Are you a British citizen?

Probably for idenification. I was asked to bring mine for an operation in the USA but I was not required to show it at the time. Usually all the information is on record.  However they do ask all kinds of questions. Whats your name? Why are you here? Do you understand what is going to happen? Just checking I would assume.

Possibly because non-UK residents try to get treatment on the NHS to which they are not entitled. Posession of a UK NHS card will prevent this.

Aren't you asked to bring your medical ID when you go for a doctor's appointment, lab work, x-rays or any other medical treatment?

We can't have people just walking in off the street, demanding to have their gall bladder removed, now can we?