Whilst not wanting to get too involved in the UMS debate, as I'm not sure of my stance, I feel obliged to point out that poor marks in either GCSEs or AS Levels don't necessarily indicate a lack of intelligence - I got a B in AS Chemistry and a low A in Biology, because I'd had a rough year due to certain personal problems. (These problems were only briefly alluded to in my reference, to my annoyance.) Yet when I was at one of my Medicine interviews, the tutor told me that of the interviewees he had seen so far, the vast majority had struggled with a certain question, and that my attempts at it were "very good". I think of everyone I spoke to afterwards I was the only one who completed it more or less unaided. Had Oxford decided to reject me outright, without an interview, because of those shoddy UMS scores, I wouldn't have had a chance to demonstrate my potential. As it was, I was made an offer.
Even if I hadn't been made an offer, I would have felt that the Oxford admissions process for Medicine was entirely fair. One incident that sticks in my mind was a question about x-ray machines at airports; when I was struggling to recall details, I was asked when I'd last flown, and as soon as they found out I hadn't been on a 'plane for three years they dropped the subject. That question was potentially discriminatory against anyone who didn't fly, for whatever reasons, but they took my circumstances into account and asked me questions I was more comfortable with.
In all of my interviews, I was constantly encouraged; when I hesitated, I was told I was doing well so far, and had I considered X or Y? Then as I expounded on X, Y, or indeed Z, I was stimulated further, and all the time told that I was doing well. I'm not being big-headed and saying I was doing particularly well, just that the atmosphere in the interviews was warm and semi-formal rather than imposing and intimidating. I said a few silly things, but the interviewers made a brief joke of them before explaining the right answer. I honestly felt during each of my interviews that the tutors wanted to see me in the best light possible.
Maybe I'm being overly generous, because I was made an offer, and I was only interviewed by six people in total so my sample size is very small! However, from my personal experiences, I have utter faith in the admissions process.