ISPs blocking it will mostly work, I think. Most people can't be bothered with a VPN so if 4chan is inaccessible they'll just read/watch something else instead.
I wouldn't put it past them, they've already ran tv news pieces (or maybe it was an ad, I don't watch tv, let alone British tv) basically saying that if you use a VPN, that you're not thinking of the children.
Seems likely. And then that won't work, and they'll tell ISPs to block VPN traffic too.