This gold-on-black embroidered badge was worn by St.John Ambulance Brigade personnel who had qualified in ARP duties. The badge was worn on the lower right sleeve of the St John Ambulance Brigade uniform. However, to date no photographic evidence of this badge being worn has come to light.
UPDATE: See this blog post with an original uniform bearing this badge.
News about interesting insignia, ARP related info and period photos that turn up.