Was there any particular reason why the Wilson government invested so heavily in a capability that it had already decide to abandon? My suspicion is it that was just to ease the pain for the RN, rather than to counter an identified threat.
More likely it was there to ease the pain for the British airframe and engines industry, who lobbied hard to get a piece of the action following the cancellations of the TSR2 and other aircraft. As noted elsewhere the RAF wanted F4s with the original engines, the rational for the Spey Phantom was the need to give the Phantom the bolter performance it needed to operate from the Ark. The result, 166 copies of the worlds most expensive and slowest F4s.