What is the most important thing to do in the interview?

From my own experience and asking other people and tutors I think the most vital thing to do is be enthusiastic and passionate about your subject. If you can demonstrate a real, genuine interest in your subject and wanting to learn it at either of the universities then they're going to want to have you. These are academics who live and breathe their subject and they want likeminded pupils studying there. I'm not at all saying let it take over your life and read an academic report a day but it's important that you do have a genuine interest in the subject and wanting to learn more about it. Knowing specific areas or subjects in it will really help towards this as well as it shows you have personal interests and have been exploring the subject.