Tbh, your A level grades are probably more important - just focus on those really. Your GCSEs may be a little below average, but there are certainly students who have got offers with lower GCSE results than you so I wouldn't worry too much about it. The scrapping of AS exams may mean that Oxbridge have to put a little more emphasis on GCSE grades as many students wont have more recent exam results, although most courses (if not all?) now have admissions tests (at both Oxford and Cambridge) so if you do well in these it should compensate for a slightly weaker GCSE performance.
As long as you're predicted at least the minimum entrance requirements at A level, do well in the admissions test and have a half-decent personal statement, you've got a decent chance of getting an interview, and from there on it really depends how well the interview goes.