The deal is it's never paid back provided you stay poor (< £20k) for your whole life. Quite a high cost for a few years of drinking.

It's a sweet deal for employers to create a natural disincentive for people to ask for raises though, while keeping the outward appearance of fairness.

It's only a disincentive to ask for a raise if you don't know what it means. You only pay back student loans based on the percentage of money you earn above a certain amount.

If you earn £21,000 and the threshold is £20,000, you only pay back a portion of £1,000, not all of the £21,000.

the £1k gets turned into ~£700.

Nobody says no to an extra £700 if offered on a plate but the lower the % you get of a raise the less motivated you are to chase it.

£910 - you pay 9% over the threshold.

In my experience, student loan repayments are, until you are way above the threshold, a relatively minor number on your paycheques; dwarfed by income tax, national insurance, and pension contributions.

Why would this disincentivise asking for a raise?

The interest rates they charge on these loans and their mission statement suggest two very different motives. Mine is approaching car loan interest territory.