How can I prepare for my interview?

In the interview you first have to show that you have the knowledge to get in there. So a good way to prepare is to repeat everything you learned in your A-levels and do some exercises (you can find everything you need by searching on Google). Even if they will probably ask you harder questions, they expect you to only know your A-level syllabus. They just want to see how you think on certain problems. After that, you have to show them that you are passionate about the subject and that you are hard-working. That means you have to talk about certain projects you did or books you read and be ready to analyse them. Try to tell them all the lessons you learned from that experience and all the skills you think you developed.