British DEMS and the US Navy Armed Guard

What was the British counterpart to US Navy Armed Guard crews on merchant ships?

Britain used naval and other service personnel in the Defensively Equipped Merchant Ships organization, commonly abbreviated DEMS.

Merchant ships carried guns, but trained service personnel were needed to operate them within naval command and ammunition systems. The British DEMS organization filled that role. This provides a useful cross-national term for readers comparing Allied merchant-ship defense.