According to the entry requirements page on Cambridge's website, keeping in mind their attitude towards GCSEs is probably shared by quite a few top unis,
"There are no GCSE (or equivalent) requirements for entry to Cambridge. GCSE results are looked at as a performance indicator, but within the context of the performance of the school/college where they were achieved.
Applicants have generally achieved high grades in subjects relevant to their chosen course, and most students who apply have at least four or five As or A*s at GCSE (7 is considered equivalent to an A in the new grading structure; and 8 and 9 is considered equivalent to an A*). However, there are always exceptions and we don’t require a minimum number of As/A*s at GCSE. One of the strengths of the Cambridge admissions system is its ability to assess all applicants individually.
Our research shows that post-16 examination performance is a much better predictor of degree success at Cambridge. While GCSE results are looked at as a performance indicator, this is within the context of the performance of the school/college where they were attained, and strong performance in Years 12 and 13 can make up for a less stellar performance at GCSE."