I only got one A* at GCSE and I got an interview at Oxford for Maths, which I believe is one of their most competitive courses. with entry requirements of A*A*A. I didn't get an offer but that was because of the interview (I asked) nothing to do with my GCSEs.
Most unis will look at your AS grades over GCSEs anyway, so if they're pretty good I wouldn't let that put you off applying to Russell Group unis!