Everyone who lives in Britain knows that it not a "common notice" as you claim.
Like wyrdolafr, I have never seen one.
To display one is a criminal offence, and conviction would carry a heavy fine.
Any Traveller who was refused entry as a Traveller would be able to make a claim against the offender for discrimination.
Far from "common" they are extraordinarily rare, and I still do not believe they exist at all.