It’s an important safeguard that proceedings should only be brought against acts which are illegal in the place they are committed. A person should obey the law of the jurisdiction in which they find themselves. They can do things quite legally in the Netherlands which might be illegal in Britain. This principle is among those which have been eroded. Aiming to act against paedophiles, UK law has been amended so that people can be prosecuted in British courts for actions committed overseas. The belief was that overseas jurisdictions might be lax in their laws or enforcement on acts the British government is determined to prevent, even if committed abroad.
It establishes the undesirable principle that the British government can regulate the behaviour of its citizens wherever they happen to be. The principle should instead be that it reserves the right to regulate the behaviour of those within its jurisdiction.