Even if Trump give Assange a blanket pardon (I'm not holding my breath) that still doesn't have any affect on what the UK government does to him. Trump can pardon him and scream at the UK all he wants to let Assange go, and the UK can just be like, "lolk" and keep him imprisoned for their own purposes.
My first guess would be to prevent a pardon from Trump. Can he pardon Assange before being brought to the US?
Even if Trump give Assange a blanket pardon (I'm not holding my breath) that still doesn't have any affect on what the UK government does to him. Trump can pardon him and scream at the UK all he wants to let Assange go, and the UK can just be like, "lolk" and keep him imprisoned for their own purposes.