The government student loan system in England effectively uses an income sharing agreement: with the most recent version, you pay 9% of your income over £25,000/year and if you haven’t finished paying it back within 40 years, it gets forgiven. This scheme doesn’t pay for itself though. (it costs the government money).
In Australia you have a loan balance based on your degree etc and pay it back with a similar % of income. I assume in the UK it's the same, if you earn 1mio you will probably not have to pay 9% (or do but can claim it back) and be done with any student loans.
Of course, but this isn't relevant to the consideration of whether UK student loans being are equivalent to an income share agreement.
If the point were relevant, then we might as well consider government spending on K-12 schooling the same way.
But general taxation is obviously not what we're talking about here. We're talking about the extra money someone pays in return for the government supporting their degree.