that it's legally a firearm in the UK.
That's absolutely correct.
Pepper spray and CS gas or CS spray are not the same thing.
Pepper spray, also known as OC spray (from "Oleoresin Capsicum"), OC gas, and capsicum spray,/ ... In Poland, hand-held pepper spray (called precisely in Polish Penal Code "a hand-held disabling gas thrower") is considered a weapon, but can be carried by anyone over 18 without further registration or permission.
In the United Kingdom, "Any weapon of whatever description designed or adapted for the discharge of any noxious liquid, gas or other thing" is a Prohibited Weapon, under S.5 of The Firearms Act 1968..