If used as a weapon then the courts could class it as a firearm if they wanted.
I'm not sure if it needs to be linked to the 2nd section to be considered one.
https://www.legislation.gov.uk/ukpga/1968/27
(b)any weapon of whatever description designed or adapted for the discharge of any noxious liquid, gas or other thing; and
[(c)any cartridge with a bullet designed to explode on or immediately before impact, any ammunition containing or designed or adapted to contain any such noxious thing as is mentioned in paragraph (b) above and, if capable of being used with a firearm of any description, any grenade, bomb (or other like missile), or rocket or shell designed to explode as aforesaid.]