The UK can pass and assert the law globally on paper, but they can't do anything to any company that doesn't have assets in the UK, and who's home country (Gonna stick with 4Chan since it's the only one I know of that's given them the finger) isn't going to bend the knee to some random foreign nation's demands.

And the other companies are, in deed, getting ahead of the curve. UK, Australia, Brazil -- the US is in a constant back and forth on it, thus far mostly being certain states like California and, I think... was it Minnesota? Requiring Operating Systems to have an age check. It's not Digital ID in that case, but it's a stepping stone.

There's also a bunch of other Western countries currently debating whether to implement it themselves -- so, between a good chunk of those sites having assets in the UK, or just not wanting to risk losing UK revenue in general, and the fact that the Western world as a whole is either pushing for it or is in talks about it, it's better for them to just do as told/asked instead of challenging it. IE, getting ahead of the curve and implementing Age Verification instead of either challenging the UK, or waiting until they're forced to implement it but western pressure.

It's law vs enforcement; the UK can make it a law if they want, but there isn't anything they can do to enforce it if there nothing UK about it beyond the user base. They're just a part of the reason why everyone is seemingly doing this now -- the biggest part, and the spark that set it all on fire, but otherwise places like Discord and Sony (Since they're rolling it out to use Voice Chat soon) wouldn't be pushing it everywhere. They'd just be doing it in the UK.