Phd student salaries in uk are probably the worst in the developed world. The student union in uk hasn’t been doing a good job to make salaries competitive with other countries at the PhD level.
Exactly. Unions end up becoming chums with the university admins. The students come and go but the admins are around a long time. Eventually the university just turns the union into a lower tier of management that effectively keeps all of the students in line. The admins learn which "issues" are favored by the unions and which issues hold sway with union. Then when they want to fire someone, they just frame it as an issue about which the union doesn't care.
Currently about 16.5k tax free. If you live in London you get slightly more, and if you're an STFC-CASE student you get more again (total of around 20k in London). It equates to somewhere between 19-22k if you were paying income tax. For anywhere outside London and Oxford/Cambridge (where rents are also extremely inflated) it's a fairly liveable wage. Shared housing pretty much anywhere else is in the range of 500 a month, possibly a bit less in the North of England. UK citizens generally don't have to pay tuition fees directly, they're all rolled into the PhD "grant".