alternatehistory.com

In the 60's the Royal Navy was in no hurry vis a vis of its carrier fleet.
In the late 50's the pair of Centaur had been modernized to handle Buccaneer strike aircrafts, in the S2 spey powered variant is was a formidable strike aircraft.
The Malta were less than a decade old, with the RN had managed to catch the angled-deck revolution barely in time, with the two Malta modified during construction.

The real issue however was air cover of the fleet : the Sea Vixen successor was to be supersonic. Soon it apeared that only Maltas could handle such aircraft, which would probably be a Phantom. By 1963 the RN had made its decision: the two Maltas would be "phantomized". The upgrade however was to be expensive at a time when military budget were slashed by Defence white papers.

In the end the axe fell on the four Centaur fleet - of two attack carriers and two commando carriers. It was decided to cut that fleet to the bone. Because they couldn't handle Phantoms, and only a limited number of Buccaneer, the attack Centaurs were retired. The two Centaur commando carriers by contrast were retained. The British government offred the two attack Centaurs for sale, but the retired Buccaneers went either to the Maltas air groups, or (later) to the RAF, which struggled and failed to get its TSR-2 off the ground.

A deal was made between Rolls&Royce and Allison for spey-powered A-7 Corsair II to replace the Buccaneers on the attack Centaurs now for sale. Then Vought noted that the A-7 Corsair II was the son of the F-8 Crusader, the only supersonic fighter able to land on a Centaur.

In the end Australia stepped forward and bought the two Centaurs by 1969, one being for spare to support the other. The "Vought deal" was attractive at a time when the RAAN fought along the USN in Vietnam. HMAS Melbourne entered service in 1973, with A-7 strike aircrafts and surplus F-8 Crusaders from the USN stocks.