Can you imagine graduating at the top of your class and you living in London or near London on less than 20k pounds a year? You can double or triple that income in other phd programs.
The only advice if one really wants to do a PhD is to sample broadly across different countries and PhD programs and choose one that unlocks future options and also pays living wages.
I disagree. I did my PhD in the UK, I did a post-doc in France and the PhD students there seem to have it just as bad, if not worse. From what I've heard from the US it's worse as well, you're expected to carry out teaching duties for pay, and your PI is more likely to string you along for 5+ years and stop you graduating since you're useful and cheap labour.
Netherlands: 3000
Sweden: 3000
Switzerland: 4000
France: 1500
Italy: 1000-1500
Spain: 1000-1500
Germany: 2000-3000
USA: 2000-3000
Uk: 1300-1600
Note in many countries you get benefits like insurance, pension, unemployment insurance (basically free money after your PhD).
In summary, there are different tiers, and UK is at the bottom tier comparable to France, Spain, and Italy.