Bob, the British Merchant Navy depended on a program (programme to you) known as DEMS: Defensively Equipped Merchant Ships. DEMS could refer to the ships, to the weaponry installed on those ships, and/or the personnel trained to man those weapons. It appears that some crew were originally from the Royal Artillery while in many other cases merchant seamen were trained as gunners. Details at https://www.liverpoolmuseums.org.uk/archivesheet71.