If I had been asked to design a means of recouping university tuition costs from the direct beneficiary I would have started by working out what the return on investment for graduate and postgraduate education actually was to the economy. Even if you needed to recover the costs without accounting for the economic gain that graduates bring through their work let alone through their higher average earnings, a graduate contribution indicated by say a G prefix to the tax code, you'd need 1.5% on the basic rate, 4% of the higher rate and 10% on the extra rate. Paid over a working life those rates eliminate the existing SLC loan book in 15 years whilst providing for new intake over those years.
Graduate tax. It's cheaper, smarter, fairer.