The same UK politicians are now pushing to block VPNs. Hence the great firewall talk which they are trying to skirt by fining US companies.

Right, but it shows their mindset. They're not letting China comparisons stop them from doing anything. It's not about the technology. In their mind, it's about the purpose and the legitimacy of any censorship.

Unlike other websites though, VPNs are generally banned from posting on 4chan, which would definitely hurt traffic.

Yes but the number of 4chan passes would skyrocket to be able to post with a VPN.