"I don't think the Swedish prison was the issue rather the Swedish extradition treaty with the US."
But the US never charged Assange (what would they charge him with? - he is not a US citizen) nor have the US authorities presented an extradition request for Assange to any country. The UK's extradition treaty with the USA is at least as strong as the one with Sweden and Britain might be more inclined to go along with " extraordinary rendition" than non-NATO Sweden. While Trump has expressed in interest in Assange, Obama's administration did not. Had he resolved everything with Sweden at the outset (it seems likely that they would not have actually taken the case to court), Assange could be long gone by now.