First, Assange is using the UK legal system to his last step, which is their Supreme Court will be his last possibilty.
Seconly, I have not herd a word that the USA has sought extradition of him, but if we were, it would be a real complicated legal set of loopholes, and could involve military laws. It is not a simple legal matter. Treaty about torture is not relevant whatsoever to Assange. Right now Sweden wants him to answer for to their rape laws, and he is fighting that right now. If the USA is trying to extradite him at the same time, what would the UK laws do in that case? Again, first come, first serve, or the severity of each extradition the prioriety.
Not speaking of Assange, if this was anyone else, Do you know what treaty may restrict extradition to either country. Case in point was Roman Polanski could not be extredited from France, but he stepped into a country where he could. Extredition from the UK could be an issue that began back in the colonial days beteen the USA and UK, I do not know for sure.
By the way, Merry Christmas to all of you down under, where Santa leaves your Christmas presents unside down under the tree, and everything spills out of your stockings, hung from the mantle and are on the floor. Even Santa loves backwards people like you!
Not your fault Austrailia is upside down.
Just kidding, Merry Christmas Ol' Friend.