I don't think there is a simple explaination, loungelizard. One factor is certainly culture - in the sort of school I teach in, many students do not come from families where parents have been educated to degree level, and therefore it's not perhaps the "obvious" thing for the students to do - in their own opinions. Also, money is a huge factor. It costs an awful lot to go to university, of course, and many students from poorer backgrounds simply cannot find the funds, or cannot conceive of spending three years getting into debt - and there's an awful lot in the media to put kids off going - all this talk of average debt and so on. Many students from poorer backgrounds would, perhaps, rather get a job, and therefore be able to contribute to the family funds.
The other thing that must be pointed out (thought it's blindingly obvious) is that most kids have to pass some sort of exam to get into private school, so private schools don't accept the kind of kids who won't get into university in the first place. I think it's over-simplifying it to suggest that "more" students from private schools go to university. A greater percetage of private school students do go to university, sure, but that's because private schools don't start off with such a mix of abilities.
I went to a top university (not Oxbridge, but RG) and there was a good mix of students from state and private schools. That was nearly 20 years ago though, and things have changed - BTECs were not seen as "acceptable" qualifications then. My concern with this whole thread, really, is that I simply think it's wrong for people of an older generation (and by that I mean academics, parents, employers etc) to demand stagnation in the secondary education sector, and to demand that students can't move on and study interesting, innovative and new subjects, because of snobbery and this insistance that newer subjects are somehow less worthy. I would be willing to bet that the same debates happened when comprehensive schools were introduced, when GCSEs took over from O Levels, and when AS and A2 exams took over from standard A Levels. If we don't allow education to move iwith the times, how can we expect our children to compete in a global market? And this whole thread is interesting when juxtaposed with another thread somewhere about how education should be catering for every individual student individually, and ensuring that they get to reach their full potential, academically, creatively and in every other way.
Sorry - bit ranty. This pisses me off though. I teach BTEC Performing Arts. All but one of my (rather small) Yr 13 class are going to university.