I picked this rather rare medal bar up on eBay recently. I've seen a few other types of these medal bars but this specific anti-gas instructor was new to me. Given that the ARPS also appears it ties in closely with the ARPS Instructor badge that civil defence personnel could train for.
I am indebted to Steve Crookes for sending me this photo of a British Red Cross medal with interesting medal bars from his collection. This shows how the above bar would have been worn. Note the 1941 ARP medal bar.
News about interesting insignia, ARP related info and period photos that turn up.