Were these aircraft involved in Operation Grapple, collecting air samples? Derek James states " four Mk 3s also participated in Operation Grapple"
Operation Grapple, and operations Grapple X, Grapple Y and Grapple Z, were series of British nuclear testing operations which sought to test a hydrogen bomb. The operations were conducted from 1956 to 1958 at Malden Island and Christmas Island (now Kiritimati). A total of 9 nuclear detonations took place during the trials and resulted in Britain becoming a thermonuclear power.
Although your last hint would possibly indicate chemical spraying.