The current talk seems to be that Sweden would have to ask the UK if they wanted to pass him to the US (not that that would be much of a barrier .. the UK government would jump at the chance to do it while Sweden gets the blame).
But what was said at the time ?
Swedish prosecutors were asked to guarantee that he would only be questioned in Sweden, but wouldn't do it. As I read it, they knew they could be overridden politically and weren't prepared to guarantee something they would be powerless to prevent. Good for them. They could have given a guarantee and then been left hand-wringing when he was extradited anyway.
Sweden had just rolled over for something the US had asked for - I forget what, maybe some copyright stuff. But it made the idea that Sweden, despite having no extradition agreement, was the US's lapdog entirely plausible.
I agree that the UK seems an odd choice. Although the rape allegations look completely unreliable on the face of it, it may well be that Assange had enough guilt about it that he wanted to run.
And then there's the bail thing. Lots of people getting apparently worked up about that, too. I wonder how many people jump bail (and disappear completely, not continue to stay in plain sight) every year. Hundreds ? Thousands ? It's an administrative offence, especially given that Assange hasn't actually kept his freedom in any practical sense. Baying for his blood because he failed to turn up in court just makes no sense when you compare it with shooting unarmed civilians and journalists for entertainment.