What PQ said is not completely true. I was an international applicant last year to the UoM for the maths and physics joint BSc course (which had an offer rate of 64%) and at the interview day I was actually asked academic questions about my application and to solve a maths problem. I do not know if this is the same as it goes for Oxbridge (as I did not apply to these institutions), but it certainly was a bit of a serious interview in my case. That said, UoM did not even have standard entry requirements for my qualification (at least they are not listed on their website) so this may be the reason why it was all so much of an academic interview for me. If you are studying A-Level or another well-recognized qualification you are probably going to be in a much better position than I was. Anyways, I eventually got a rejection after the interview day, thus they do reject applicants in some circumstances. Wish you good luck.