> How do european (or perhaps just the UK's) legal system(s) work in that regard?

It's actually European law that's binding here, and extradition / deportation denials based on human rights concerns are nothing new - there have been bans on deportations even to EU members such as Greece (https://www.gesetze-bayern.de/Content/Document/Y-300-Z-BECKR...) or Hungary (https://www.keienborg.de/2018/02/19/die-12-kammer-des-vg-due...).