Probably early in the war, this photo shows dignitaries being shown around the Marconi Wireless New Street factory in Chelmsford, Essex (possibly this factory). The helmet and armband markings are specific to this factory's ARP team. The gentleman on the far right has an armband that features MW and TC (combined) - Marconi Wireless Telegraph Company - and CONTROL added to it. A previous MW armband appeared at auction and can be seen on this blog post.
Additional research provided by members of the WW2 Civil Defence Re-enactors FB page.
News about interesting insignia, ARP related info and period photos that turn up.