Positive discrimitaion isn't fair, I agree with the idea that with grades such as AAB, the student from the poorer performing school should be taken (note that some private schools aren't worth the money you pay, and that some state schools acheive really good results, it all depends on the catchment area).
However if you have two candidates with straight As, what can you do? Both students couldn't have acheived more. Either they make the a levels harder (which I'm all for), or they bring UMS scores into account.
As for ethnic minority discrimination, I'm not sure, I supposed if it was the situation like I illustrated in my first paragraph, then that would be ok. But you need to take other factors from their background into account.