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It will be up to porn sites, it seems. They have to check age on visitors who come from the UK or else they will face penalties. Those that don't will presumably risk being caught up by the existing bodies which monitor the internet, which obviously use bots and other things to sweep around looking for this stuff.
Of course, if you know what a VPN is and how to use it, then getting around the age check is fairly trivial. Which does rather raise the question of what the point of it is if not to harvest data on people.