I think it's hard to say as most people applying to top Unis will have relatively good GCSEs anyway. However, there are a few people on here who have got into great universities without the usual string of A*s and As, so it's possible.
I think universities accept that at age 14-16 young people often haven't developed/matured enough for a set of results to be held against them for the rest of their lives. AS results and A2 predictions are certainly viewed as more important; although places like Oxbridge do apparently consider your GCSEs to a greater extent than most.
Basically, what it comes down to is supply and demand. If UCL receives 2000 applicants for 200 places; they will probably narrow things down by AS and GCSE results, and then look at personal statements etc. BUT, a weak-ish set of GCSE results isn't going to necessarily *stop* anyone from getting into a good uni provided that they have good AS levels and predictions.