Haha, my interview is in 12 days! Regarding ethics, I believe it is slightly different for every interview but some interviews may have ethics questions, it's really hard to give a definitive answer. The focus of Cambridge interviews is, as you said, academic. In fact, it is mostly all talking about science (which, I don't know about you, but really excites me!) However, they like to throw in the odd variation e.g., they might ask you a bit on your PS, work experience etc. maybe an ethical problem. The great thing is, you can steer the conversation the way you want it by mentioning what you are interested in/have most knowledge about. So if, like me, you find ethics to be a vast ocean of contradictions and 'Oh no he didn't' traps, just don't mention them!
I will probably prepare for medical ethics by buying a book, seems the best thing to do. There is a veritable buffet of ethics books on Amazon, Im not really sure which is the best, but it is perhaps an idea to look at comments etc. when deciding
Good luck, hope that was helpful and to which college are you applying? Also, I have two interviews- one with two interviewers, and another with three! Did you mention yours is a one-to-one? LUCKY