Something tells me, that Jaguar-M as Phantom replacement (in other words: as air defence fighter) would suck. It could be an improvement over French Navy's F-8 Crusaders, but step backwards from Royal Navy's F-4. In terms of capacity, I think that only improvement over Sea Vixen it would have would be being supersonic.
There's a reason that the MN dropped both the F1 and Jaguar as naval aircraft. They really were ill-suited to a maritime conversion... Buccaneers were good at what they did and materially good until the early nineties so sod getting rid of them for something such as the Jaguar.
The problem with RN F-4 Phantom was the they had "wrong" engine. The Spey was larger from J-79 and structural changes in F-4 to accept Spey frame affect aerodynamics. British F-4 Phantom had a bit smaller wings, less speed and range from US ones, because the Phantom was designed around GE J-79. I said this before, if i put Phantom in this AU Britain they would have same electronics with real F-4K but the J-79 engine. Also, Britain could attain license to assembly the airframes.
What other choice Britain had except the F-4? British design, from those some pages back, is preferred.
The Spey was IMO the better engine for the Phantom. Top speed is an irrelevant figure which was governed by as in all British projects lack of monetary funds to do the job properly. The Spey was a more economical and cleaner engine than the J79. In retrospect had the funds and will been there the Spey probably would have ended up a lot better off than the compromised position it finally found itself in on the F-4.
There really isn't a viable alternative to the F-4 in the fleet defence role until the Hornet comes along (at least one that would launch and trap on British carriers)