It will depend entirely on what each side can offer the other, to get what it wants, out of the arrangement, in return. If Pakistan revokes his nationality then UK cannot render him stateless. So that may not be likely as a practical solution.
It may not come to even that. Applying an additional punishment for a crime for which the sentence has already been completed, by a retroactively applicable amendment would not be easy to get through UK courts. If it ever comes to that.
Your personal views, understandable as they might be, have no role to play in the provision of justice for all, equally.
After all, it would take ignorance of epic proportions to claim that a "free man" who "has served his sentence" somehow does not deserve anyone even "advocating on behalf of this individual and his right to exist freely", because that is utterly antithetical to the very concept of justice.
He committed a crime. He was caught and punished by due process. He has served his sentence.
Any civilized society with rule of law would, and should, find it difficult to go beyond that as a matter of principle.
It takes courage to stand up for what is right, no matter what, not only when it might be convenient to do so. Only ignorant fools follow where their blind emotions might lead without control, since that is the recipe for chaos.