Free Gary McKinnon
This principle is internationally accepted. e.g. the International Criminal Court is controlled by this principle.
But again, the ship needs to have be physically present when piracy took place to be "hunted down and hung".
It's a very simple principle of locality and nationality: You can only be prosecuted if you break the law of the land where you are in OR break the law of the nation you belong to. You can not be prosecuted for any action which is legal in the land and legal in your own nation.
The usual basis of extradition is, if the act is also illegal in the country where it occured.
Breaking into the computers of the British Government is equally illegal, but the act was not against them, so except the silly stuff about no death sentence, which is local protection, the guilty are sent to where they commited the crime.
Hence, you cannot be extradited from the Grand Caymans for tax evasion, for they have no tax laws. For murder, you will be on the next plane.
There is no doubt that the computers involved were in the US, so that is where the crime occured.
You do not have to be present to commit a crime, only be the cause of it.
England has laws about counterfiting, their own money, so if you counterfit American money, it might not be an English crime, but even if you passed it in Europe, it would still be a crime against America.
He cannot be tried in Britian because he did nothing there, also, they have no idea where he went, what he looked at, and what he downloaded, or planted, only the people running the American Security that caught him know. His entire path through is known, computers leave a trail, and it lead back to him.
He says UFO, but another word is antigravity, lots of countries would like to know how we do that.
Where he went, what search terms he used, will tell as much of the story as we will let out. What he saw will not even be told to the judge.