"That is a sociological fact."
There's a cheap gag there, of course, but let's leave it on the table.
"far more students from far lower down the socioeconomic scale have gone to university"
Have they? Or is what's really what happened is that institutions that they would have gone to anyway were relabelled universities, while courses that they would have done anyway were relabelled degrees? In the 1980s, the choice wasn't university or t'mill, there was a huge range of full time, block release, day release and evening education for everything from ONC up through to HND and degrees in non-university settings. Polys, FE colleges, slightly longer ago Colleges of Advanced Technology (Aston was definitely one, I think Loughborough and/or Keele were others). These were rigorous, nationally validated qualifications delivered in a huge range of settings, and gave both academic rigour and employment opportunities.
But now, all of those institutions are universities, and they're delivering degrees to roughly the same people, covering not dissimilar ground, but now they aren't nationally validated, and the students have to pay a lot more for them, and because they're full time concepts like day and block release don't arise.
Further up the scale, what were once slightly declasse redbricks are now desirable Russell Group universities, and if memory services the socio-economic mix has got less diverse there, not more. University take-up has increased, but to a large extent by relabelling qualification that weren't (and, let's be brutally honest, in many cases still aren't) first degrees and places that weren't (and, again, still aren't) universities.
Yes, there are going to be some people who would have had no post-16 education back in some anti-golden age, but get degrees now. But they're not the main part of the shift, I would venture.