Because they can be arrested and extradited to UK. Not a high chance, but not zero. And a realistic one is to be arrested while traveling in Europe and extradited to UK.
Extradition treaties, such as the UK–US Extradition Treaty of 2003, allow for extradition requests between the two countries.
They generally require that the alleged offense be a crime in both jurisdictions ("dual criminality") which ensures that individuals are not extradited for actions that are not considered crimes in their home country.
Extraditing someone because their website uses encryption would force them to prosecute so many people and organizations it would be a joke. You would have to really piss them off.