How are they going to police and enforce it ?
If a porn site is based in the States or Russia how is the UK going to force them to comply when it's out of their jurisdiction and also how will they prevent UK citizens from accessing if they can't enforce ?
And even if it does become enforceable surely it's just easily skipped over via a VPN ?
Not to mention what about more adult oriented social media sites like VK which is littered with porn and people posting porn to even twitter ?
I'm sure though the government will get their cut of the profits from this glorified porn tax which is likely the main motivation behind it, with the excuse being "protect the vulnerable" which funnily enough never offers any kind of protection
This restriction of things never works because curious minds want to know why it's restricted, how about providing better education on topics like porn & drugs to young minds instead of trying to hide it away from them for as long as possible making them want to have access to it to see what all the fuss is about in the first place.