I had the same concern as you before I applied to Cambridge, I was paranoid about my GCSEs (as they're pretty rubbish: A*A*AACCCC - 1 of the C's being in Maths, ugh) and when I visited the college that I applied to, I asked the Admissions Officer about them, and I said "I understand that GCSEs are important too, and I've only got a C in Maths, will that be a problem?" She actually laughed! And said, no, of course not - as long as I got good A-level results, that was the main thing.