A graduate tax is a reasonable idea, it wouldnt have put me off going to university whereas £7k tuition fees would have.
However, the likelihood of them ringfencing the money is not high, and there would be no link of the graduate tax paid and the university attended, so no rewards/incentives for better performance from the universities.
Also it will be a headache to administer, and putting a tax on a good degree, is like charging those who use the NHS more … both are intended as free at the point of entry, but because you have a degree and then a good job you should pay more? Then why not charge those who use the NHS more, and those who use libraries more … etc etc
Instead, an increase in income tax, could fund higher education. It would truly be accesible to everyone if their academic abilities were high enough. It would also be very simple to enforce minimising administrative costs.
Justification for this … it is not just graduates who benefit from the degree, society benefits when they become teachers, lawyers, doctors and it is the tax higher earners pay that funds facilities from which we all benefit. Business is crying out for more graduates, if the economy wants the graduates … and the graduates will benefit the economy … its an obvious solution.
It would need to be managed, numbers attending would need tight control …