Hey, I had an interview for Biochem last Wednesday. I was pretty nervous too! The interviews are quite informal, but it was harder than some other interviews I've had because my interviewer asked me more questions about science than he did about me. And I could talk about myself for hours... so I had to try my best to recall some basic facts from AS Bio and Chem. They might well ask you what you find most interesting about Biochem, so make sure to do some background reading to cover yourself if they try and delve deeper. Like I said I particularly liked genetics and he asked me "What do you like about it?" And I was a bit like "...um."
Don't be nervous, the interviewers aren't trying to catch you out, just find out why you chose that course, that uni (that's a likely question, "Why UCL?") and then he also asked me about interesting practicals I had done (had to do some quick thinking). He also asked me questions that I didn't know the answer to (don't panic!) for example I was asked what a cyclic ester is called. I mean, is that on the A-Level syllabus? No. So I didn't know, and admitted that. Just appear interested and enthusiastic.
My interview went ok/quite well... as long as you don't make a pigs ear of it you should be ok! I got my offer through yesterday =D
Oh and another thing to relax you; in the opening presentation, the head of department, Chris Taylorson, said that most people that reach interview get an offer. So relax, you've almost made it! Good luck! =D