Technically, he is committing a firearms offence.
"Its is an offence for a person to have in his possession any firearm or imitation firearm with intent
1. by means thereof to cause, or
2. to enable another person by means thereof to cause, any person to believe that unlawful violence will be used against him or another person."
S.16a Firearms Act
Definition of an 'imitation firearm' S.5(1)b Firearms Act "anything that has the appearance of being a firearm whether or not it is capable of discharging any shot, bullet or other missile".
Personally I would have nicked him to prevent a potential breach of the peace and dumped the little shite the other side of town. The Officers showed admirable restraint.