It's a tough one. If your parents can afford to send you to an excellent school, ply you with private tuition, and make sure nothing gets in the way of your learning, you're going to have a good chance of top marks. If you go to the local comprehensive, study in poor facilities, have the local chavs setting fire to things, get no extra tuition out of hours, and have to take a part time job on which means you can't study on certain evenings, then I don't think anyone can unequivocally say an AAB student in the latter scenario isn't Oxbridge material while the former is.
If it were up to me, I'd look at things on a case by case basis, perhaps in a similar (but better way) to Durham, where points are scored for those who do so much better than the school's average. At at least two of the Scottish ancients, Glasgow and Edinburgh, preference is given to the locals, and its not unheard of for people from the local school with four Bs at Scottish higher being accepted but an ABB student at A-Level being rejected. It's more of a recognition that some people don't have the luxury of picking a university and going there, and if the A-Level students can afford to come up from England and live away from home, then they sometimes have to take second fiddle to the arguably inferior student who can look no further than their own city. This system hasn't been complained about, so I don't see why a case by case examination at Oxbridge should be either.