Just wear whatever feels comfortable. I wore a quarter zip and formal trousers, some were in suits, some wore jeans and a T-shirt. As far as the interview, it just depends who interviews you to be honest. My interview was a very relaxed conversation about a practical I did on electromagnetism, particle physics on topics like wave-particle duality, dark matter and dark energy. Most of the talking was done by my interviewer as he explained various different topics we were speaking about in detail, and I asked a few questions about the university and what they were looking for in a student. Other applicants had one A-Level style question to answer (which I didn't), and some had a few. But they also said that they were quite easy to answer, so that shouldn't be a cause for concern. Overall, the interview day was really interesting and informative. They also say that if you have an interview they will already be making you an offer, but the interview is to be used in the next summer if you miss the entry requirements by a grade. Hope this helped.