>the government doesn't like the idea of displaying an official page stating that you are not allowed to see something

They don't even have to do that, the connection can just be left to time out on the client side. This is what they did (and some ISPs still do) for the Internet Archive...

Yes, archive.org is classed as an adult site in the UK.

The UK is a funny place. 4chan is accessible but IPTorrents is blocked.

Browsing with a VPN is a frustrating experience. They are abused by many of their users which leads to circular capture checks and straight blocks.