Strictly speaking I believe that if the defamation is published in the UK (including available on the internet and one person in the UK reading it) then the UK courts might still have a go even where neither the author or the defamed are UK based.
It is however an expensive path to bring a case (and defend one) and damages aren't always massive so it isn't really worthwhile unless you have money to burn scaring people away from telling stories (true or not) about you.
It's worse than that: in the UK, some statements are libel even if they are proven true, simply because the damage to reputation is so great that the truth of the statements is irrelevant if the facts were not public knowledge prior to publication.
For example, that guy who got caught in a Nazi-themed sex dungeon...
