Hi Earlofthegrey :-) Try not to get too bogged down in the details of the NHS, it is so complicated that I suspect most of the people working for it don't understand it fully! In addition, questions about the NHS rarely form a huge part of interviews. I remember my King's interview and we didn't talk about it at all.
That being said, a basic grasp would be useful just in case and the basic points given by HMarles are an excellent foundation. The only thing I'd add is perhaps about some of the core principles of the NHS (e.g. free at the point of use) and how this differs with other health systems across the world. In addition, read up a bit on any current affairs in health that are happening around your interview.
Best of luck,
Alex, 4th year UCL medic
6med