I think he should be prosecuted under UK law, he commited the alleged acts in the UK, he never placed his foot on US soil. Virtual crime does not have physical jurisdiction. Even recently, there was a FBI case involving several nations where the responsible were charged in each nation under local law.
The only thing an extradition would serve would be to try to scare off would-be intruders to government networks. Most people know that its just a fantasy that does not work since only the careless people get caught, and the only reason to go after someone abroad is to make an example that "we can reach anyone" because a bunch of morons forgot to secure their systems and lost face.
If they spend the money on securing their systems instead of pointless lawyers...
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"It is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring" (Carl Sagan)