Warwick has got a lot more competative, but my brother is there doing Physics and although he got 3 As at A level (this was before A*), for GCSE he got 2A*, 3As, a B and 4Cs. Granted, he says that he has lower GCSE results than all his friends there, but his A*s and As were in Maths and Sciences, which is what is most important for his course. If he was applying to do English (which he got Cs in), I doubt he would've been accepted though. As long as the good grades are in the relevant subjects, it shouldn't matter as much.
EDIT: Also, can I add that he is on track for a first in his degree, whereas one of his friends who was more of an all-rounder, didn't get through to the 4th year of the course. Basically, say for a Physics degree, just cos you're good at English, it doesn't make you any better at Physics and I'm sure top universities will know this.