"...if you write something for a local audience and some entity in a country you can't find on a map decides it is a punishable offence..."
Easy: if the document viewed was stored on a server in the UK, assuming the alleged act is actually considered a crime there, then the applicable jurisdiction should be the UK.
This is directly comparable to, say, a Frenchman in Calais reading a poster through a telescope pointed at Dover and finding said poster offensive. The poster itself is in Dover, in the UK, and therefore subject to British law, not French law.
I would consider viewing a website on a server in the US as analogous to flying over to the US to browse through a book in a Seattle bookshop, before returning home. All we've done is eliminate that tedious mucking about with baggage handlers and airplanes.
Now, you could argue that the website's data is processed and displayed on the user's computer in his own country, and thus is (temporarily) stored in his computer's RAM—and probably cached on the hard drive too. But this is a personal copy, not the original data. If anything, it's the user who is responsible for commanding his computer to make said copy, so if he has broken his own country's laws in doing so, *he* is responsible for doing so, not the original website.
Under the Computer Misuse Act 1990—I doubt the Terrorism Act 2000 applies here—McKinnon could be sentenced to up to two years, or a fine, or both. (12 months seems the most likely.)
As UK law does not apply in the US (and vice-versa), the US is also quite capable of trying McKinnon "in absentia" as well. If the prosecution wins, this would likely prevent McKinnon from ever entering the US—he'd be arrested upon his arrival at the port of entry. Not only to Americans thus never have to worry about McKinnon taking advantage of the lucrative lecture circuit, but they also save themselves the costs of his incarceration.
All that said, McKinnon has himself admitted to accessing a computer in another country without permission. His supporters cite "Aspergers" as mitigation, but I have an ASD myself and I certainly don't go out of my way to break laws and perform unethical acts as a result of it. He's clearly not fit to use a computer, so I would at least ensure that certainly never happens again. You might as well give a pyromaniac access to a box of matches and the keys to a fireworks factory.