I've had interviews at Imperial, UCL and King's and they were all slightly different from each other
Unlike a lot of other people, my Imperial interview was quite hard. They asked me some questions you would expect at a medical school interview (for example, why medicine, about teamwork, communication) but also quizzed me a LOT on health economics as I did Economics for A level. The lead interviewer challenged everything I said and tried to pick holes in my answers, particularly when I gave my suggestions on how to solve obesity and smoking - at one point, he said that my idea would result in me killing all poor people in the country (which, of course, threw me considering the pressure of the interview situation). I think they were just trying to see how well I could justify my ideas though
UCL also asked me some stuff about health economics but not to the same degree as at Imperial, and the interview felt less of an interrogation compared to Imperial. In general, I enjoyed this interview the most out of them all. At the start there were a few normal questions - the standard why medicine, pros/cons of being a doctor, communication. There were some challenging questions asked but they led into these questions quite well and it felt more like a discussion rather than an interview. They also asked stuff about my BMAT essay - what I thought could have been improved and then asked me to explore a few things I said in my essay a bit further.
My King's interview was hard - the interviewers really grilled me hard and challenged pretty everything I said. They also picked out small details in my personal statement and asked me a lot of questions about it. They make interviewees sign a document saying that we cant discuss the content of the interviews so unfortunately that is all I can give you
Overall, I would say that interview experiences can be variable at all three universities. Almost everyone I talked to about the Imperial interview said that it wasn't too bad, and even the people on my interview day said that most of the interviews were OK. Maybe I was just unlucky and got a harsh panel? The story was similar at King's - lots of people said that the interview was okay but again, I must have got the harsh panel on that day. At UCL, some people said that their interviews were really hard whereas others said that it was okay.
Therefore, be prepared to face a hard interview - even though others may have had a slightly more relaxed one, you may get a hard one!