This plug and receptacle is technically known as the BS 546 (5 A/250 V earthed).
This plug was originally defined in British standard BS 546 and widely used in the United Kingdom until the late 1940s. It has three large round pins in a triangular pattern, and is still commonly found in countries originally electrified by the British. This type was also previously used in South Africa, but has been phased out there by the larger, higher-amperage Type M plug. Similarly, in some former British colonies, the plug has been mostly replaced by Type G design. This 5 Ampere plug, along with its 2 Ampere cousin, is sometimes used in the UK for centrally switched domestic lighting circuits, in order to distinguish them from normal power circuits.