From Wikipedia:

Neil John McCallum (1929–1976) was a British-Canadian actor. After attending the Guildhall School of Music and Drama in London, McCallum’s first major appearance on stage was alongside Sam Wanamaker in The Rainmaker in the mid-1950s.[3] He appeared in British TV series in the 1950s, 1960s and early 1970s, including The Saint, Department S and UFO, and in films such as The Siege of Pinchgut (1959) and Dr. Terror’s House of Horrors (1965).[4] He provided the voices of Dr Ray Pierce in the film Thunderbirds Are Go (1966) and an airport controller in the TV series Captain Scarlet and the Mysterons (1967).[5] He played lead character Angelo James in the BBC TV series Vendetta (1966-68).

He was also a scriptwriter (scripting two 1964 thrillers, Do You Know this Voice? and Walk a Tightrope, among other things), a producer and occasional director.