yellowstone is right, it is subject dependent, but even so Boffinmum clearly works in one of the less fussy subjects. Keble College (Oxford) provide admissions feedback by subject on their website and below are some quotes Link Here:
Economics and Management (one of the very most oversubscribed courses). In the list of reasons why candidates might not be invited to interview it said:
"Results in official examinations, especially GCSE, which are not at the highest level ? candidates should have virtually spotless GCSE results to secure an interview."
For Engineering: "Almost all candidates have all or mostly A* at GCSE and predictions of straight A grades at A level"
Law: "Almost all candidates had all or mostly A at GCSE and were predicted a run of A or A grades at A level."
Theology: deselection if: "results in official examinations, especially GCSE, that fall significantly short of the minimum achieved by successful candidates in recent years, virtually all of whom show a run of consistent A or A grades at A level and predominantly A at GCSE."
MFL - No mention of GCSEs talks only about language abilities.
Physics - deselection based entirely on PAT results (ie: GCSE independent)
and so on - pretty much the full range of GCSE results needed from perfect to doesn't matter, but very dependent on the subject chosen.