He joined the Royal Air Force Voluntary Reserve (RAFVR) in 1939 and was commissioned as a pilot officer. But his height (6'3") and poor eyesight meant he could not be a pilot.
Wikipedia describes him as "a flight lieutenant in the Royal Air Force who had been seconded to MI5, Britain's domestic counter-intelligence and security service. He had been appointed as the secretary of the Twenty Committee, a small inter-service, inter-departmental intelligence team in charge of double agents."
Wikipedia has this photo captioned "Charles Cholmondeley and Ewen Montagu on 17 April 1943, transporting the body to Scotland."
Erenow gives an account of Charles Christopher Cholmondeley's life after the war: "In October 1945 he joined the “Middle East Anti-Locust Unit” as “First Locust Officer,” a job that involved chasing swarms of locusts all over the Arab states and feeding them bran laced with insecticide", going on to suggest that this was a cover for the work he was still doing for the British secret service. He was appointed MBE in 1948. This intelligence work also took him to Malaya but he left MI5 in 1952, moved to the West Country, married Alison, and set up a business selling horticultural machinery.
The plaque describes him as a British aristocrat which seems likely but we can't corroborate.