Two people are walking through a metal detector on the London Underground. One comments to the other that it’s “a piece of shit that wouldn’t stop anyone”.
Result: they are stopped by police, searched, and charged with an offense under Section 5 of the Public Order Act, which prohibits “Using threatening words or behaviour likely to cause alarm, harassment or distress”.
In other words: pointing out stupid ineffective security might distress the sheep, so pass a law and fine anyone who does it.
Since the 2000 Terrorism Act, UK law has classed as terrorism anything which “involves serious damage to property” or “is designed seriously to interfere with or seriously to disrupt an electronic system”, so long as it’s done with the intent of influencing government. That makes for an awful lot of terrorists; and the kicker is section 13:
A person in a public place commits an offence if he-
(a) wears an item of clothing, or
(b) wears, carries or displays an article,
in such a way or in such circumstances as to arouse reasonable suspicion that he is a member or supporter of a proscribed organisation.
So if your T-shirt causes “reasonable suspicion” that you support an organization that has caused damage to property with the intent of influencing government behavior—like, say, Greenpeace—you can be fined, put in prison for 6 months, or both.
There’s more. Under section 19, if you become convinced that someone else has committed such an offense “in the course of a trade, profession, business or employment”, then you are committing a crime unless you tell the police about the suspect “as soon as is reasonably practicable”.