They did ask me what gp practice I was with.
Same here. We had to complete a questionnaire. There was one foreign woman in the waiting room who was called up to the desk and asked for proof she was able to use the NHS - as she had stated on her forms that she didn't have a GP practice.
I didn't hear what the outcome was, though, b/c I got called up for my appointment.
So it's not a given that they'll just treat you.
I think in larger cities, they're starting to twig about healthcare tourism.
We recently switched surgeries and I had to show my NHS card for the first time ever.