The UK previously didn't allow extraditions with the death penalty as an option, or extraordinary renditions, whereas Sweden allowed extraordinary renditions.
However the UK has recently changed it's stance on allowing death penalty extraditions. https://www.theguardian.com/uk-news/2018/jul/23/uk-will-not-...
I might be mis-reading the article, but it doesn't indicate that the policy on requiring pre-extradition guarantees against the death penalty since the people in question were never extradited from the UK in the first place.
As they're no longer British citizens, were arrested in Syria, and are already in the US awaiting trial, it doesn't seem the UK would have much basis to argue against prosecutors seeking the death penalty.
It’s a bit like avoiding jumping out of the frying pan and into the fire, but neglecting that the frying pan is full of oil until the oil catches fire and then being surprised that you’re on fire.
And then saying “see, I told you about the fire”.