It's not a tax. It's repayments of a debt on a loan. The "debt" increases massively year on year because of the punitively high interest charged. So those who don't pay much of their debt off in the early years suffer a much higher interest charge, and pay the loan repayments for longer, than someone who takes a better paying job at first (or repays their debt from savings). Of course, some people pay nothing off their debt, and lots of others pay back the original loan (capital) but never pay off all the interest. But, at the end of the day, it's a debt, interest is charged, and repayments are made. So it has all the elements of a loan and few, if any, elements of being a "tax".
It is neither loan nor tax, however some would say more tax than loan. Martin Lewis uses the term "contribution" as a working compromise.
So the SLC rate is now 6.3% against inflation of 10%, and rising. That's a negative real interest rate. What is the commercial rate for an unsecured loan? At least 10% and up to 30% at the moment? Interest starts accruing at the same time and does not freeze at the end of the course. So SLC "loans" are relatively good value compared to every other funding option.
To an extent none of this matters, nobody needs to engage with the SLC if they have a better option. The question was whether the amount should go up to £24k from c.£9k. If there had not been a freeze, it is estimated it would be £12k now, and presumably rising fast with inflation. Vice-Chancellors have mentioned the £24k that internationals get charged, perhaps a crude negotiating opener? I can't see the any government agreeing to that.
Whatever it is, having a flat fee rate means humanities students subsidising STEM, if they contributed equally when working. However in general STEM graduates contribute more, having higher pay on average, so perhaps it's swings and roundabouts, and we can pretend that a flat fee rate resulting in say an Literature student (cost c.£20k) having the same "loan" balance as a Medicine student (cost c.£200k) is the best option.