They are a 'made' offensive weapon created for the sole purpose of causing injury to another. Therefore, being caught in possession of such item in public is the offence. The only points to prove that the CPS need to gain a prosecution is that you had the item in your possession in a public place. They do not need to prove intent of what you were going to use it for as it has no other use.
If caught in possession in public by police, you will be arrested and stand very very little chance of escaping with no further action.