I went to an Oxbridge Access Summer school last week, and the admissions guy from Cambridge said "the average Cambridge applicanthad between 4 and 8 A* GCSEs." However, he said that they do accept people with significantly lower grades.
Apparently they focus more on GCSEs now as they did before as they cover more of the academic subject spectrum - basically not just subjects the student likes.
I'm not sure about individual colleges though, that's just a general answer.