AEW for the Invincible Class was very much an add on as a direct consequence of the Falklands War. The ships had been designed as Helicopter platforms to operate a viable number of ASW Sea Kings, primarily in the North Atlantic, the Iceland/Faroes gap or off Norway. Of course an even earlier add on had been the Sea Harrier/Ski jump with the SHAR mission being to "hack the shad" in the form of Soviet LRMPA that could provide over the horizon targetting data and/or fire off ASM's at NATO warships.
In the 1970's nobody thought we would go to war in the South Atlantic, well away from land based fighter or AEW cover and in a non NATO context. When we did the SHAR was really all we had to defend the Fleet in conjunction with SAM's, few of which were suitable for use against low flying fast air. The Sea King AEW was rushed into service but only in time to go aboard HMS Illustrious when she sailed south after the wars end to relieve HMS Invincible in Summer 1982. That was pending the construction of a runway long enough to handle RAF Phantoms, allowing them to take over Air Defence of the Falklands later in the year.
The danger of cutting your cloth by making bold assumptions about what you will need in future is always that of being wrong and then having going off to war with the wrong kit. Plus ca change!