Just as the USA in the case of IRA terrorists
There is nothing special, in this regard, about the USA. Such a principle as you espouse must be true for all countries vis-a-vis each other. The problems come when each country has a different standard of legal system (probably news to you, but the US standard is very variable and even within the USA is often seen to be far from meeting normal standards of impartial and fair justice and certainly differs greatly from that of the UK, including Scotland). In addition, just because it is against, say, a French or American law, does not mean it is against a British law or a criminal rather than a civil offence.
This kind of thing is usually condemned as one country trying to extend its reach into another. The fact is, whatever your opinion, the alleged offence, if it is an offence under British law, was committed in Britain. If, for instance, a person is alleged to have killed a foreign national in Britain, do you think he is tried in Britain or in the land of the foreign national?
Now, in fact, this was just a somewhat fanatical hobbyist exploiting a weakness in a computer system. Nobody has published some detailed account of how the costs of the damage were calculated, nor of what this damage was. I should love to see you, after alleging someone has damaged your property, running to the police to lay an information about damage of £x without some detailed justification of your claim and calculations.
It is a fact of national independence, that people can behave within a country in ways that may be detrimental to another country. In Britain, every week I should think, somebody burns an American flag or damages some Chinese property in protest about Tibet or tries to damage the reputation of Israel or whatever. In many countries, these things are criminal offences. By your lights, that is enough for the perpetrator to be extradited even though, here, it is normal freedom of expression.
Or are you telling us that the USA is special because it is big, powerful and does not really execute people, inflict ridiculous punishments of hundreds of years of imprisonment, be almost openly corrupt with judges chosen for their political allegiances and notorious abuses at local court level. What kind of system gets prisoners, not yet found guilty, dressed in orange overalls and treated as guilty?
If McKinnon committed an offence under British law, by all means charge and try him here. If not, that is the end of the affair. Otherwise, there are an awful lot of intelligence agents, industrialists, journalists, protesters, writers etc. who should be being extradited to the USA, Russia, China, Iran .... I wait with baited breath for that Danish newspaper to report the extradition of its journalists to some Moslem country to be punished for publishing "blasphemous" cartoons, in contravention of Pakistani or Saudi Arabian or Turkish law.