Sure, 4chan is a cesspool, but what if I start a replacement? How does the UK block it? Do we end up with an allowlist only internet?
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You'll only be able to connect to domains that have been bought with a state-issued ID and digitally signed. If you run afoul of the rules, you'll be taken down, fined, or worse.
The means to publish and consume will be taken from us.
"Trusted" computing. "You wouldn't download a car." "Think of the children". "Free speech allows hate."
Within a generation of complete and total control of communications, we will be slaves. Powerless, impotent, unable to organize, disposable fodder.
1984 is coming.
The convention is use DNS to resolve domains and DNS providers play by some rules, but if enough people start to dislike the rules you will start seeing unsanctioned DNS services and the like.
Another option is for browsers to consult a p2p DHT (just use the one for torrents) for a special class of domains (eg. https://[pubkey].dht). This is similar to how Tor does this but in this case you don't need to hide your server location because presumably it's located somewhere where the laws favor you.
IP blocking is a very different type of problem and one that would require hiring China as a consultant. And still be only marginally effective.