As already said I believe Oxford probably won't consider you, Cambridge *may* if you do amazing at AS/A Level and get good references etc. However, at other Russel groups (eg. Nottingham, Exeter and others, check their websites) they do this thing called a Contextual Offer. There is basically a list of schools and colleges with below average GCSE results where they *may* lower your university offer based on the fact that you did well in a situation that was very challenging. However, I *think* this only applies if you remain at your current school (rather than changing to a college, unless that college *also* is below average for GCSE results). I'm about to start applying for uni so you may want to check this with someone who has applied and gotten a contextual offer.