"The Research Report: The economic benefits of a degree published by Universities UK in February 2007, reports the average lifetime earnings of a graduate as £160,000 more than those of a non-graduate with two A-levels. Within this average there is a range from £340,315 for medical and dental graduates to £51,549 for a humanities degree and £34,949 for an arts degree."
Doing a degree in humanities or arts now involves a net lifetime financial loss, assuming students graduate with fees/costs of £30,000 and interest to be added over decades. But of course, the actual fee+cost will be rather more than that. So why bother?
Of course, if people do science degrees instead, that won't increase the pool of science jobs, so the same number will now be taking those poorer paid non-science grad jobs previously taken by arts and humanities grads (but with their more expensive and largely irrelevant science degrees).
Society needs people to do those graduate jobs. But it seems we now want them to pay for their own university education at a cost greater over their lifetime than they'll gain by doing those grad jobs rather than non-grad jobs.
[[P.S. actually I exaggerate as those who earn less will never pay full amount back and it is written off after 30 years.]]