The Crown Prosecution Service has been reluctant to authorise actions against hard core porn web sites under the Obscene Publication Act. They say juries do not want to convict.
That being so, the answer is obvious...Create a new regulatory offence. Web site owners would be required to show they had a robust age verification mechanism in place. Not having one would be a crime...but, because most of the owners of the porn sites in question reside overseas the penalties for the proposed new offence would have to be sufficiently severe to allow extradition treaties to be invoked, to bring NON-Brits to the UK to face trial in our courts!
annaraccoon.com/2014/07/23/the-minister-for-global-porn/