Under what law is Britain obligated to extradite him?
You might not know, but Assange has not even been indicted in Sweden. He is wanted for QUESTIONING. There is no warrant for his arrest.
And just to keep things totally in perspective here, the first prosecutor in Sweden threw the case out for lack of merit. It took a second, more politically 'connected' prosecutor to revive the case.
It is a fraud. Assange is a political enemy, nothing more. In this time of universal deceit, he chose to tell the truth. Indeed, as Orwell pointed out, that was a radical act against the establishment.