The interview itself isn't scary at all - I had mine last year. It was literally a 20 minute chat with Dr Rolland (you can find more information on him on the Exeter University website), about the law in general, my A level subjects etc. I didn't have to do research beforehand, although it helps to have an idea of what the course will entail. Try to talk as fluently as possible, and just try to be calm and natural - that's really all the advice I can give.
However, what you need to pay more attention to is the degree itself. As cumu1813 says, it is obviously difficult. Not many English students can speak French to the kind of level required; you're basically thrown into the French university system. There are only 10-12 students to start with, which probably eventually lowers to around 5 (from what I've heard, at least). This is why you need to be 100% confident that you want to do this degree (it isn't offered at many places), and that you are intellectually capable and hardworking enough to do so. I love France and would love to gain some experience there, but realised this degree isn't for me - particularly because I can't see myself living anywhere other than England. However, because you eventually want to move and live in France to work there, this degree would be a great way of launching you straight in and seeing how you'd cope in that sort of situation.