Well, the first thing to realise is that although grades play a very significant role in the selection process used by universities, it is not
all they use.
So if you were to establish, for example, that 90% of students who were predicted A*A*A* received offers, it wouldn't tell you why the other 10% weren't. (It could be that those grades weren't in the right subjects, or there was an issue with their GCSE Maths and/or English grade, or their personal statement may have been very week, etc.).
Similarly, if you were to establish that 10% of students who were predicted below AAA received offers (even though the standard offer was AAA) it wouldn't tell you why. Perhaps they were contextual applicants and had experienced challenges during their education, or perhaps they represented their country at the Olympics and the university wanted to make allowances for the impact that would have had on their education.
The above is an attempt to put grades into context.
Universities tend not to advertise the grades that accepted students actually achieved, as they consider the information commercially sensitive. As a result, the information you're looking for isn't always easy to find.
Some universities will publish admission statistics on their web site which go down to grade level, so check the university web site for this data. For example:
For each course / university combination,
The Uni Guide lists the "Most popular A-Levels studied (and grade achieved)", which can give you some clues. For example, if we look up Computer Science at Warwick (
here) we can see that they ask for A*A*A, but that the "Most popular A-Levels studied (and grade achieved)" are Mathematics A*, Further Mathematics A and Physics A. That suggests that they ask for A*A*A but more typically accept candidates with A*AA. But the source data behind this statement is not available, which is a shame.
You could also look at the spreadsheets the TSR users create to track received offers. This data is clearly not statistically valid, but you might be able to tell (for example) is anyone has receive an offer when their grades are below the typical requirements. For example, the user-generated spreadsheet for 2023-applicants to Bath is available
here.
Finally, you could look for previous Freedom of Information requests for this information on
https://www.whatdotheyknow.com/, or even file a request yourself.