The UK law is stipulating adult content can only be viewed if you are provably over 18. They are putting all of that responsibility onto the websites/platforms to enforce that.

If a child goes to a shop and tries to buy a pornographic magazine and they are denied, is that censorship?

If a child tries to see an 18 film at the Cinema and is denied, is that censorship?

The fact is both of these were freely and easily done on the Internet as most websites do not verify a users age.

I do not like the online safety act as it is, but it is not "censorship".