Asd 400 is a lot a month and as you say it really does not add up. Maybe if I can grow this £ they could do both.. Save some money on fees, maybe half... And have money for house deposit etc..
This really would not make sense. You are not understanding how it works. For a start you only pay £400 a month if you earn £79,000 a year. And you would pay the same monthly amount no matter what you owed. So if you owe £80,000 you pay it for about 23 years. If you owe £60,000 you pay it for 16 years. Realistically is your child likely to earn that much for 16 out of the next 30 years?
A new graduate is probably not going to earn anything like that for the first few years. They may pay nothing off at all for 10 years. Even if they have a well paid job, they may pay a bit then have a family and never earn enough again.
Plus even if they do end up in a well paid career, they can spend that £20,000 on a house whilst they are still earning less at the beginning of their career. If they avoid a student loan they won't see any benefit in take home pay unless they earn quite a bit. Meanwhile they will be trying to save for a house deposit on fairly low earnings, whilst spending on rent. And may need a bigger more expensive mortgage.
In my case out of the 15 years since I graduated I did a PhD for 4 years, took maternity leave for a year, then went part time and will probably earn barely more than the earnings threshold for several years yet. Currently my loan is the same as when I graduated as the interest is the same as what I've paid off. During the 9 years that I've worked full time I earned between £23000 and £35000. I fully expect to still owe the same in another 15 years when it will be written off. So whether I owe 60,000 or 80,000 makes no difference whatsoever. It would have been a complete waste of money to reduce my loan by £20,000.