a us person could travel to a country x and that x could send this us person to a UK prison? I don't know if doing so would be legal but when the rubber hits the road, each country x is technically sovereign and does not have to honor the first amendment of the US constitution.
So even if it might be frowned upon to extradite foreign (US) nationals in country X (such as Canada or India) to the UK, they could do it anyway to send a message?