I have been involved with admissions at Oxford for chemistry (but have sinced moved on), and I can tell you a bit based on my experiences there. First of all, different colleges have different ideas about what they're looking for. In my experience they're looking for top quality candidates (A levels are really only considered, GCSEs didn't really have any impact at all) who can adapt and excel in the interview too.
Certainly at Oxford, how you perform in the interview is what sets you aside from other candidates. The interviewers are looking to push you out of your comfort zone and see how you cope with questions you can't possibly know the answer to. If you can reason your way through, that's a really good thing. The A level grades are what get you to the interview - how you perform in that is what gets you the place or not. GCSEs might get looked at by some, but I never bothered, and no-one I knew did either.
People develop at different times. Two years before my GCSEs, I was in the dunce's maths class. By the time I took GCSE maths, I got an A*, then 100% in two of my 4 A-level papers.
Interestingly, I got my worst GCSE marks in Chemistry. I went on to get a 1st from Oxford in Chemistry, and have since done a PhD in Pharmacology and Chemistry (I don't consider myself a Chemist any more, by the way).
Don't know if that helps or not, but I thought it might be useful to share my experiences from the other side of the table, so to speak.
Roo