Found this:
Francis FitzGibbon QC
If he fears that the Swedish prosecutor is merely a cat’s paw for the US government, who will demand his extradition when he arrives in Sweden, he has to take account of Section 58 of our Extradition Act 2003: if the Swedes want to extradite him to the USA, they have to obtain the consent of the British Home Secretary first. That rule derives from Article 28 of the 2002 EU Council Framework Decision (2002/584/JHA), which binds the Swedish government. After the investigation and any criminal proceedings in Sweden end, that restriction also ends and the Swedes can extradite him without reference to the UK government. But neither Sweden and the UK will extradite anyone to a country where the accused is in peril of the death sentence if convicted of an offence, or where prison conditions are so bad as to breach his rights under Article 3 of the ECHR (‘No one shall be subjected to torture or to inhuman or degrading treatment or punishment’).
ETA: Added bold to above quote.
To my mind, you didn't bold the bit that's probably exercising Assange. That's the second bit that I've bolded. It's possible to imagine a scenario where the Swedes rapidly complete the investigation, conclude that there are no charges to answer, and then they're off the Article 28 hook and Assange is on the next plane across the Atlantic.
Why they can't just extradite him direct from the UK I don't know; perhaps our government thinks it would be bad politically to do that and the Swedes don't, but to extradite a rapist to Sweden doesn't have any political ramifications.
I'm fairly certain that Assange will end up in the USA at some point, but whether this is really a strategic game to get him there sooner rather than later, who knows. I certainly don't.