In the early 1960s I don't think the RAF were even thinking about it. The Lightning force was still building up, and the Bloodhound SAM was also shiny and new.
How much consideration was given for the Phantom to take on the air defence role en masse when it was purchased in 1965 I am not sure; certainly 43 and then 29 had new AD Phantoms from the start, but the remaining squadrons all had hand-me-downs. Had the P.1154 entered service with Strike Command, however, there would have had to have been procurement of a dedicated AD aircraft as it was not at all suitable for the job. Possibly a variant of the RN P.1154 would have been chosen to replace the Lightning in the mid-1970s- whether the Navy had bought it or not- but the Air Staff may have sought a clean sheet design or considered the F-4E, which was still very much in production at St. Louis
Alternatively the RAF may have looked to procure the new fighter that was taking shape at the same factory, the F-15, bravely choosing to run the Lightning on until the late 1970s/early 1980s. With an early order the RAF could have been fielding an operational Eagle squadron by 1979, and that would have meant, of course, no Tornado ADV either.
Whether of course the RAF could have afforded enough F-15s to replace the Lightning outright is another matter, and I suspect that there would still have been two Lightning squadrons retained in the UK, and maybe even the two RAFG squadrons. This may well have led to a more urgent need for what became the Typhoon, and another deviation from what actually happened.
Solve one problem, and another springs up...