Isn't it a Radway Green-made ARWEN 37 mm round?
As mentioned earlier, either four-element CN, CS or smoke.
Three element versions are still produced, such as the AR-2 and AR-4 series:
AR-2-CN-SR3 ARWEN Pyrotechnic CN Irritant, Short Range Three Canisters;
AR-2-CN-LR3 ARWEN Pyrotechnic CN Irritant, Long Range Three Canisters;
AR-2-CS-SR3 ARWEN Pyrotechnic CS Irritant, Short Range Three Canisters;
AR-2-CS-LR3 ARWEN Pyrotechnic CS Irritant, Long Range Three Canisters;
AR-4-W-SR3 ARWEN Pyrotechnic White Smoke, Short Range Three Canisters;
AR-4-W-LR3 ARWEN Pyrotechnic White Smoke, Long Range Three Canisters;
AR-4-R-SR3 ARWEN Pyrotechnic Red Smoke, Short Range Three Canisters;
AR-4-R-LR3 ARWEN Pyrotechnic Red Smoke, Long Range Three Canisters;
etc., etc..
As to its exact UK designation, hmm don't know?