The UK routinely extradites people to the US (and facilitated extraordinary renditions from UK soil). The claim he could not leave the UK for fear of being extradited to the US was always a nonsensical lie.
It is perfectly normal - If the judge orders the person's extradition, he must remand the person in custody or on bail pending the extradition. He was granted bail by the High Court in London and was released at Stansted airport during the afternoon, where he boarded a plane and departed the UK.
In reality he is not "free" till the judge slaps their hammer down.