I had a life sciences one today. There were quite a few people in suits which made me laugh a bit but the majority was smart casual. During the talk we had at the start the women said they'd chopped half the applicants they received and we were very very likely to receive an offer.
The interview wasn't too bad it says in the interview booklet you're not going to be asked any academic questions and you're not. My interviewer just asked me about my courses at A level cause they are a bit of an odd mix, why i choose they and what i like about them. He asked me what i thought the course was about as again it's an odd one (Biological and Computer Sciences). Then i asked him quite a few questions, i dropped something in that apparently they don't do till 3rd year so he seemed a bit impressed by that, it was preempted by him though. I did get a bit dazed however when he went into detail about some stuff but just nodded and smiled lol. Then he pretty much went through my offer, they're not supposed to say you will definetly get one just infer it, we went through it in a bit of detail though, "What subject would you like you B grade to be in?".
That was pretty much it. Try not to be too nervous the interview was a pretty nice chat, i did fall over my words a little when trying to explain some stuff.
Oh also, ALOT of people came with their parents, so as long as you don't mind being on your own initially you should be fine.