Earlier quoted context omitted.
They’re not breaking any law. Laws apply to actions in the country, they’re not based on citizenship. If you go to Amsterdam and sleep with a hooker, you didn’t break a law by doing that: despite prostitution (specifically purchasing sex) being illegal in many western countries.
Commonwealth countries have extraterritorial jurisdiction. I don't know that it's ever been enforced for something so relatively petty as intoxication or prostitution, but it is nevertheless the law. (Obligatory IANAL though.)
No countries have extraterritorial jurisdiction.
Some countries have a lot of influence over the local jurisdiction outside of their own territory.
The UK doesn't have much influence like that.
But if the UK has any minesweepers, I bet this could all be sorted out with a few phone calls.