First, had been arrested in Sweden before fleeing jurisdiction. He would have been sent back to Australia, long before any extradition request would have been sent. So bollocks to that.
Second, even if he had gone back to Sweden from the UK, again the UK could have nixed any extradition such that he would still have gone back to Australia when it was over.
That said, Australia would have handed him over 'toot sweet' [sic] Meaning that he was already arrested for hacking US systems while a kid so he has a record in Australia and the complaint would have been a second offense against the US. Not to mention the US and Australia are on good terms.
So if the US wanted him... they would get him via the Australian Government.