Mike Tregenza I know very little about. He was an illustrator of educational books for children in the 1970s, contributing a number of titles to the 'Picture Panorama of the World' series published by Owlet Books (1977) as well as books on space travel and the planets. The earliest book I have spotted was written and illustrated by Tregenza in 1974, entitled Espionage. He also produced book jackets and an excellent set of illustrations for a series on the history of the submarine for Look and Learn in the spring of 1981.
(* Illustrations © Look and Learn Magazine Ltd.)