Botany Bay should do nicely
For the UK to lock him up is legit, though IMHO a bit petty at this stage.
For Sweden to put him on trial and lock him up if found guilty would be entirely legitimate. But he's a hot potato, and I suspect the Swedish authorities would rather never see him again. Hence why they haven't issued a new warrant.
For the US to lock him up would be political, and political prisoners deserve our support. The prospect of supporting Assange has very little appeal.
If Sweden shows no interest, just let him go when he's served his sentence: we routinely do that with criminals at all levels of seriousness. Or deport him back to Oz - the sooner the better.