Oh is see it is the porn site itself blocking the uk visitors

Sure. A little more to it than that though.

AFAIK the UK most likely said "this content is illegal, if you don't remove it, we'll go all Kim Dotcom on your ass" and it's entirely up to the website operator how they want to respond to that. They could fight it like Kim did... might not end well though.

Any large website with user uploaded content receives notices like that routinely (including Lemmy) and most of them respond by deleting the content itself, because it's usually pretty nasty shit or else it wouldn't have got the feds attention. It's treated the same as malware/spam/etc.

For whatever reason, these ones chose to take the entire website offline if necessary.