When is a joke a terrorism offence?

This joke can be found in the current edition of Private Eye:

According to police officers in Leeds, this joke is an offence under the Terrorism Act:

Thanks to the arrest of the man holding this placard, the above image is now circulating much more widely in the media. The Guardian alleges that an officer at the protest said, ‘If it’s in print, it’s fine. But if you carry it on the demo [it’s illegal]’.

For the benefit of anyone who is comedically challenged, this is a joke. The inverted commas around ‘PALESTINE ACTION’ make clear that it is not referring to a certain recently proscribed direct action organization. The point of the joke is to highlight the hypocrisy of a government that criminalizes opposition to war crimes in Gaza while turning a blind eye to those who are perpetrating those war crimes.