The classic 40mm Bofors cannon was clearly obsolete versus high speed jet aircraft by the 1950s. The Royal Navy could ill afford to equip most of its fleet with expensive, and large, guided missile systems such as the Sea Slug missile system. So Shorts Brothers developed the lightweight, relatively inexpensive SeaCat missile system in the early 1960s.
A small, lightweight, subsonic missile using Command to Line of Sight Guidance, SeaCat could be mounted on just about any warship, and was for many Royal Navy ships the only air defense aboard. The gunner used a set of binoculars on a pedestal mount to track the target, and the guidance system relayed steering commands via a radio link. Theoretically, as long as the gunner kept the target within the crosshairs, the missile would guide to the target.
It was also widely exported to the usual British client states.
In reality, the system, while very reliable, was not terrible capable. During the Falklands war in 1982, out of over 80 SeaCat firings, only one Argentine Skyhawk was brought down. Interestingly, the Argentinians also used SeaCat and its land based variant, Tigercat, though they scored no kills.