If you travel to Spain and rob a bank there you are subject to Spanish law. However if you flee back to the UK before they catch you, Spain then has to ask the UK to extradite you. Different countries apply different approaches to extradition and different terms and conditions. Some don't get extradited at all (think the US woman who killed the teenager through driving on the wrong side of the road but flew back to America.)
In the case of you robbing a Spanish bank, you would likely be extradited fairly easily. If say it was an African country accusing you of spreading homosexuality then no because that would clash with human rights law. But the UK is fairly reasonable about extradition.
The Dutch are more awkward, and apply more conditions. It's an issue even outside of Van de Velde since until the start of this month it wasn't illegal to groom kids online for example. So your Dutch nonce could be online talking to a child about sex in another country. Technically he is committing a crime in that country (since the communication is also happening there). But good luck getting the Dutch to extradite.
There's a reason it's a major hub for child porn.
My personal opinion is we should have agreed to send Van De Velde (and others like him) back to the Netherlands but made him swim tied to a large millstone.