Its Not A Two Way Street!
This goes far beyond Gary McKinnon. There is a much bigger picture to see here.
Firstly, at the moment, the USA can extradite anyone they wish from the UK. They don't even need to show that a crime has been committed. They just need to say that someone in the UK is a suspect in a crime, and there is nothing we can do to stop ourselves being carted off to the USA.
Secondly, you can be extradited to the USA, even if you have not committed a crime in the UK. For example, there is a couple in Scotland, who sold chemicals, perfectly legally, over the internet, to all the countries in the EU, including the UK, and beyond. Because someone in the USA bought a certain chemical which is banned in the USA, they are in the process of being extradited to the USA for supplying said chemical.
Thirdly, the UK can not extradite a US citizen under the same circumstances. The USA dont not have to show any kind of evidence when extraditing a UK citizen (a mere allegation is suffice), but the UK still needs to show "probable cause" requirement in the US when seeking to extradite someone from the USA.
Lastly, the UK authorities had the opportunity to try Gary McKinnon in the UK under the Computer Misuse Act. They declined to do so. Had they tried him in the UK, no matter if he was found guilty or not guilty, he could never have been extradited to the USA.
Everyone in the UK was sold down the river by Blunkett and New Labour, when he signed this extradition treaty. A treaty, which i might add, was not subject to parliamentary scrutiny until months after it was singed.