Yes, you’re right… some people don’t actually pay anything at all if they’re below the earning threshold. I was only thinking about those who pay significant amounts of the loan and forgot that some people won’t pay anything or very much.
The interest rate varies according to inflation (it’s the same for everyone), but the amount paid back each month varies with earnings (basically, the more you earn, the more you pay each month). The issue is that it clears the debt quite quickly for high earners with a small amount of gross interest whereas a moderate earner will be paying a smaller amount over 30 (or 35? I forget now) years - because they’re paying off less each month and the interest mounts up over time, they’re charged more interest and pay off a larger total sum than the person who cleared their loan quickly, but arguably could have afforded to pay more of their income and benefited more from their education.
It’s perhaps not as regressive as I first said, but it is still a regressive system when set against something like a graduate tax.