Short answer: A level grades have nothing material to do with achievement in a degree. In that once a student is admitted, we really don't care what their A level grades were.
But it's never that simple is it?
In my field, we spend most of the first year undoing a lot of the damage done by current national curricula, teaching to the exam, and the treadmill of achievement and learning being measured by exam results.
This is not teacher-bashing - I know this is not teachers' doing & that most good teachers find the current politicisation of A Levels and School curricula as damaging as we do.
In three/four years of an UG degree, students do a lot of growing up, maturing, personal growth etc. So we would hope that they learn way way way beyond their A Level studies, and realise the adult world of knowledge & independent thinking.
In fact, I can pick a recycled A Level essay from 100 feet - one of the 1st year courses I teach overlaps a bit with some of the cognate A Level syllabus. If students go back to their York notes (or whatever) they will not do well.
Basically, University teaching - and more importantly learning - in my field (and I'd bet Humanities more generally) is miles away from, more complex than, and more demanding than any A Level.