Molio You don't have to believe me of course, but what 'everyone knows' about admission to good universities is often very far from the truth.
I know, for example, that some of the universities you listed as 'hard hitters', make offers to people that have Ds at GCSE (obviously not in a related subject) and have dropped from ABB to BCC in clearing to fill their places on a range of subjects (and not ones deemed to be 'soft').
I only know this as I work in admissions and, later on in the application process, we can get data on which universities were names as firm or insurance where people also applied to us. I also see when the A level results come out how far universities drop for applicants (where we are the insurance) and at what level universities go into clearing.
Remember, what universities say in public about admissions is also part of their 'brand identity'. So if you came to one of my open days, and asked me about what GCSE results we would be looking for, I would tell you that whilst we look at the whole application, we like to see evidence of exam success. But when I am making offers, you need to pass the threshold level (5x Cs including English and Maths) but it plays no other role in the application process.
There are some institutions that are very keen on GCSEs, Bristol is one. There are also a tiny minority of subjects (eg Medicine) where they matter. But these are a tiny minority, rather than the majority.
When AS levels go, GCSEs will probably be used more again. But this is a bit unfortunate as our data indicates that GCSEs results are not a good predictor of degree outcomes (which is exactly why we don't really use them).