In the UK it's illegal to carry a weapon. Anything made as a weapon; eg a knuckleduster, adopted to be a weapon; eg. a stone in a sock, or intended to be a weapon is illegal.
To prove you intended to use it as a weapon police are trained to question you along the lines of "if an attacker came at you; would you defend yourself with this?"

If you ask them "well would you?" and they answer yes. You can make a citizens arrest if they are holding it, they just admitted committing a crime :-).
qjs