Martin Lewis does explain it well. But with the loans for students starting now (there are several earlier versions of the loan with different repayment terms), the low salary level for starting repayments (barely above full time minimum wage) and the extension of the repayment period to 40 years, means that more students will be repaying in full, albeit over 40 years - so their entire working life.
When first introduced, lots of people thought they wouldn’t be paying back or they would only start paying back when they had a reasonably well paid job. This is no longer the case. The IFS estimated that 79% of students will repay their loans in full under the new system (compared to 49% under the old system). Obviously how much you will pay back is a guesstimate because no one knows how much they will be earning. This is a good tool for playing around with the numbers and assumptions:
https://www.student-loan-calculator.co.uk/
Most people will obviously need to take the loans, but they should also recognise what a burden they are. The “real value” of the loans now grows in line with RPI, but in absolute terms it will be a very large repayment if it is spread over 40 years.
I know no one will be crying over high earners, but there are lots of stories of for example medics with incomes of £100k, which we might think of as very high, but with very large students loans, loss of child care credits, loss of child benefit, mortgages/rent etc, who are finding themselves left with very little money indeed. Marginal tax rates (if you think of the student loan as a graduate tax) can be 80-90%!
I’m not saying that there shouldn’t be tuition fees, but I don’t think people should assume that they probably won’t pay them back or will only pay them back if they are well paid.