I've heard that in Slaughters interviews they sometimes ask whether you were rejected from Oxbridge (I was). How would I go about answering that question without sounding like I was really unprepared for my Oxford interview etc.?
Well if you think it's going to be a problem, unless you've mentioned it in your application for some reason (which would be quite weird if you didn't go, imho) just don't bring it up. Ultimately an interview is your chance to talk about your experiences, and it follows that you should be able to choose which ones you mention, and vice versa which ones you don't!