Interviews at college are generally informal chats, I was just asked about things I wrote on my personal statement, then was asked about what things i wanted to be involved in at uni, what i want to do in the future and why i chose the college.
The department interview (for computer science) was focused on the subject. Things like... what you think you will gain from studying it, why you chose it, and recollection of any experience in the subject came up. Then I was asked some basic CS and Maths questions, asked what I thought I would get at A level and was then told i would get an offer.
The interview process seemed pretty informal, you really don't need to prepare... i didn't anyway. My interview at Durham was the same as interviews at other unis, so that should give you some idea if you've had one before.
hope that helped.