During the Second World War, Exell's knowledge of Portuguese – he was also fluent in French and German – led to his being seconded to the Government Communications Headquarters at Bletchley Park, and working as a cryptographer in Cheltenham. Returning to the British Museum (Natural History) in 1950, he founded the 'Association pour l'Etude Taxonomique de la Flore d'Afrique Tropicale' (AETFAT) and started the Flora Zambesiaca project, the flora covering the catchment area of the Zambesi River, which at the time comprised the Federation of Rhodesia and Nyasaland, Bechuanaland, the Caprivi strip and Mozambique. He visited the region in 1955 with Mendonça to make further collections. He was co-editor of Flora Zambesiaca from 1962 onwards, and was awarded a D.Sc. by the University of Coimbra in Portugal in 1962. In 1971 the Portuguese government conferred on him the 'Comendador Da Ordem De Santiago da Espada'. He also received the Order of the British Empire in 1961, and was a member of the Linnean Society of London and other scientific societies.
