This helmet is currently for sale on eBay (at time of writing bids had already exceeded £175). The seller claims that it is a first world war vintage helmet that was reissued in the second world war. It has 1938 dated chin strap lugs.
Derry & Toms was a London department store on Kensington High Street founded in 1853. It was renown for its roof garden. It closed in 1973 and the building was subsequently developed into smaller retail spaces and office space.
Derry & Toms is also Cockney rhyming slang do bombs, as in "A Tom dropped on my house..."
News about interesting insignia, ARP related info and period photos that turn up.