I note none of those justifying tuition fees have indicated whether they'd be up for tuition fees for sixth formers,

I do not justify tuition fees, but we are broke and it mostly benefits the well off.
Only those who become wealthy and successful repay significant amounts.
Do they need handouts from those who never went to university.
With limited resources, there are more deserving cases.

Also we are producing far more arts degrees than the country needs.
That is a bad investment for scarce capital, and does not even promise a good future for the graduates.