kirinm If I borrow £10k from a bank to buy a car, they will expect me to pay it back monthly over an agreed term. If I get sacked or diagnosed with a serious illness next week, the bank will still want it's money and short of going bankrupt or a formal IVA etc they will still expect me to pay them.
Not the same at all for student loans. A new graduate could spend 30 years earning a low wage, or not working at all and pay back not one penny of their student loan and then it will be written off with no hassle whatsoever. Not all will spend their working lives in graduate jobs. They may work part time and earn less than £21k, they may be unable to work for any number of reasons or may spend a lot of time as a SAHP.
Student loan repayments reduce your monthly income in the same way as taxes, your salary goes down, your graduate tax student loan payment goes down.
Yes, you might be able to borrow slightly less for a morttgage, but that's not necessarily a bad thing, as maximum income multiples can often be unaffordably high, and cause difficulties if circumstances change.
And just like living in London as a working person might not always be affordable, it's the same for a student. It's not like its always the best or only option. If you don't want to take on extra debt, don't choose to study in one of the most expensive cities in the world.