The Spey Phantom is an interesting aircraft in this perspective. Britain couldn't afford to develop a Phantom class aircraft in the mid 60s, but managed to get considerable sovereign capability, wartime supply assurance, buying things in Pounds rather than USD by insisting on some 40% British components which incidentally solved several technical issues.
I think the UK could have developed a Phantom class aircraft if the POD was 1962 and it was done instead of the P.1154 project.
I have some notes that say the Spey Phantom cost double the estimate to develop. That is the total R&D cost rose from about £50 million to about £100 million. This included the engine development cost which rose from about £25 million to £50 million. Unfortunately, I didn't write down the name of the book that I read it in, but I think it was Francis K. Mason's book. Development of the Spey Phantom took longer than estimated as well.
However, to be fair to the people that decided to buy the Spey Phantom, projects like these often take longer than estimated and cost more than estimated.
A British Phantom class aircraft could have cost more than expected to develop and taken longer than estimated to develop.
The TTL version of this website would have many threads arguing that it would have been cheaper to buy the Phantom instead of developing the Hawker Siddeley Spectre, which is my name for the British Phantom class aircraft.
Having written that I think the £21 million spent on the P.1154 to 1965 and the £100 million spent on developing the Spey Phantom IOTL would have gone a long way towards developing a Phantom class aircraft between 1962 and 1970.
I have also read that the changes that were made to make the Phantom capable of operating from Eagle & Ark Royal and the 40% British components produced a non-standard aircraft that cost considerably more to build than "ordinary" Phantoms. The devaluation of Sterling in November 1967 increased the manufacturing cost further, but the planners that decided to buy the Phantom cannot be blamed for failing to foresee that.
Therefore, I think a British Phantom class aircraft would have cost about the same to build as the Spey Phantom.
A British Phantom class aircraft might have performed better than the Spey Phantom because the airframe and engine could have been designed to fit better. AIUI fitting the Spey to the Phantom spoiled the area ruling and that degraded its performance.
Similarly, it might have had lower take-off and landing speeds, because it was a new design rather than an adaptation of an existing design and it could take advantage of the advances in STOL technology that had been made between when the Phantom was designed and the early 1960s. Therefore, the British Phantom class aircraft might have been able to operate from the British aircraft carriers that were capable of operating the Buccaneer.
Finally, a British naval aircraft of the 1950s and 1960s usually folded into a smaller package than the equivalent American aircraft. Therefore, a one-to-one substitution of the Sea Vixen with British Phantom class aircraft aboard Hermes and Victorious might be possible. That wasn't possible with the Spey Phantom even if it had been capable of taking off and landing with a useful payload.
That is, provided there isn't an A.B.C. (Another British Cockup).