I've applied for Physics with Theoretical Physics and I had my interview on the 16th Nov. It was quite informal. There was a group of 10 (I was the only girl) and first we had lunch with a couple of undergraduates and they showed us round the college and some of the accommodation and answered some questions, then we were split into two groups and we had a group interview where the lecturer asked us about why we wanted to do physics, and talked to us about what's available on the course and why Imperial is so great. Then he showed us round some of the different research groups. We then had the individual interview, where he talked about a few things on my PS such as my work experience and a few of my extra-curricular activities. Then he asked me about wave-particle duality and we takled about Young's double slit and the photoelectric effect. That was it really.
At the end he said something about the offer being 3 As, and he did say "hopefully see you next year or something" but I still haven't got an offer so I'm not sure whether he will or not!
All in all, not too scary, but after you visit you'll really want to go!