Where would you suggest he should go?
UK courts have a long history of letting extradition cases take years and/or denying extradition, and UK courts have a history of reacting to political pressure by standing firm rather than yielding. There are few places where I'd trust the legal system more.
Which other countries do you think have courts willing to stand up to political pressure, and where the US can't or won't just pick him off the streets without causing a major diplomatic crisis, and that would be somewhere you'd be able to get visas and where you'd want to live?
I can't think of many countries that'd fit those criterias. Most of the places I can think of where I'd believe the US would be unlikely to be able to get at someone are places I'd rather not want to live.
But the UK would be high on my list of places where I'd feel the risk would be low.
I have heard people say things like "Sweden has co-operated with NATO in [....]". Maybe Sweden does co-operate with NATO, but it's not a member and the UK is.
The UK courts might seem wonderful in merry old England, but just look at what happened in Northern Ireland, and you'll see a different face of the British legal system. Interment without trial, torture, extrajudicial killing, all of UK citizens on UK soil.
http://www.cnn.com/2012/10/16/world/europe/uk-us-mckinnon-ex...
The UK has a history of both courts and the Home Office denying extraditions. Theresa May is a "law and order" authoritarian to boot, but even she is not prepared to fall over herself to satisfy US extradition requests.