People are replying to my point that fewer people should study A levels and access tertiary education with very bizarre anecdotes.
Of course, talented mathematicians should go on to study Maths and talented musicians study music (though, judging by the musicians I know, it is pretty thankless in terms of a career for the vast majority).
But anecdote doesn’t make data. Most who aren’t strong at GCSEs aren’t good at A levels either. The boards publish the mapping of grades in each subject and even those getting 7s are unlikely to achieve better than a B at A levels either. And if you achieve a B at A level, university shouldn’t be for you! Again, obviously there are exceptions but they are few and far between.
As for classes mixing at uni, they don’t really! Most middle class people choose other middle class people to mix with and marry. One of the exceptions is at the top universities, which are selected on intelligence and potential. There is far more class mixing there. And they wouldn’t be affected if only 10-20% of people went to uni.
In fact, free uni, and even bringing back grants in scarcity subjects (at least) would encourage far more poor but talented students into uni.