The Porpoise class was the first class of operational submarines built for the Royal Navy after the end of the Second World War, and were designed to take advantage of experience gained by studying German Type XXI U-boats and British wartime experiments with the submarine Seraph, which was modified by streamlining and fitting a bigger battery.

[1] Propulsion machinery consisted of two Admiralty Standard Range diesel generators rated at a total of 3,680 brake horsepower (2,740 kW), which could charge the submarine's batteries or directly drive the electric motors.

Sealion attended the 1977 Silver Jubilee Fleet Review off Spithead when she was part of the Submarine Flotilla.

[6] In late 1986–1987 Sealion was deployed to the South Atlantic, carrying out patrols from the Falkland Islands before visiting Chile and returning to Britain via the Caribbean.

[8] She was paid off in December 1987 and sold to an Education Trust for deprived inner-city youngsters "Inter Action", arriving at Chatham on 22 June 1988.