I don’t agree with the legislation, but I assume the same way they can go after you for any crime they perceive as being committed in the UK: extradition. It’s obviously incredibly unlikely.
Source: I was recently listening to a podcast about court cases and an extradition case came up. They mentioned that reasons to not be sent are mainly about human rights, like if you need medical treatment that the target country can't (or isn't likely) to give, or if you would have to sit out your punishment under inhumane conditions (oftentimes you can sit out your punishment in the country that extradited you after having stood trial in the other country's legal system), or if the country you're being sent to was involved in your torture (them handing you over to another party that you know does torture would count, even if their own hands are 'clean'; that was the crux in this case, whether the victim aka accused person was handed over by the united states to pakistani intelligence and the usa therefore had forfeited their claim to further try the person for the alleged crime)
Or, you travel to the UK! It’s a pretty popular destination and Heathrow is a major European hub. It would be easy to get caught out.
The law may well be onerous and misguided. But looking at that site, it seems they reeeeally should do their due diligence. Not just to avoid the long arm of UK justice, but other territories too. It looks extremely dubious.
Their mitigation doesn’t make sense either. If they don’t shutdown UK accounts and those accounts use UK credit cards which continue to use the service via a VPN. It could be reasonably argued that they know they’re providing a service to a UK resident. So, they really need to do their homework.
What they’re actually complaining about is the cost of doing business. It sounds pretty amateurish.
When did this happen?