CS spray. White crystalline powder in an solvent propellant. About twice as concentrated as military tear gas, designed to to cause immediate and extremely painful incapacitation through irritation of the mucosa, trained to deliver in a short burst direct to the eyes nose and mouth. Effects last up to 20 minutes, most symptoms gone in about 40 minutes. The cop using it on this clip failed to shout "spray" before he used it, which is why his mate didn't know it was coming and got a faceful as well. He does shout "gas, get back" after he used it, but that's not much good either for his mate or his own credibility. The gasped "Nice one Jason" from his colleague was, I feel, dripping with theatrical irony.
It is indeed a section 5 prohibited firearm, needing authority from the secretary of state to own it. You are as likely to be granted that authority as you are to get a rocket launcher, covered by the same legislation.
Trivia fact: CS Doesn't work on fish.