'P' and Phantom refers to ....
GHQ Liaison Regiment (known as Phantom) was a special reconnaissance unit of the British Army first formed in 1939 during the early stages of World War II. The regiment's headquarters were at The Richmond Hill Hotel in Richmond, Surrey (now in London); its base (including the officers' mess and billet) was at Pembroke Lodge, a Georgian house in Richmond Park, London.
David Niven (the actor) was in the Phantoms...The story has it that was why he chose the name 'The Phantom' for the jewel thief he played in the Pink Panther films after the war....Ian