The UK does not have stricter rules regarding extradition than Sweden. That's a meme that germinated from a revision to the extradition rules that closed a gap between the US and the UK in which it was easier for the UK to extradite from the US than vice versa.
Moreover, Sweden can't extradite without the UK's approval in this situation, so the whole issue is moot.
> Moreover, Sweden can't extradite without the UK's approval in this situation
I don't mean this to sound combative, because I am really just curious about how practical the distinction is: Can't or shouldn't? What would happen to Sweden if it neglected to wait for the UK's approval and just stuck him on a plane to the US a day after he arrived?
You said a lot of things that lead me to believe you're probably not an expert in international law, so I'm going to exercise my prerogative not to take your word for this.
I'm not an expert. But consider what happened in the rendition case. The penalty imposed on Sweden was
"granting monetary compensation; permitting a new application for asylum in Sweden; and legislative changes prohibiting the use of diplomatic assurances."
Sweden paid less than half a million dollars to each of the two victims. The US would gladly reimburse Sweden 10 times that amount.
Moreover, Sweden can't extradite without the UK's approval in this situation, so the whole issue is moot.