The only reasons to pick the Crusader over the Phantom by the mid-60's is if your carrier is not physically capable of operating a Phantom (France) or as an austerity measure.
In hindsight, arguably the RN is better off buying off the shelf Crusaders than pouring money into redesigning the F-4 to accomodate Speys, but that wasn't remotely apparent at the time when the RN still had sights on bigger carriers. It was better than the Sea Vixen needing to be replaced, but the bar is pretty low. Financially, it's a lot cheaper than redesigning the Phantom to use for 10-15 years.
The more redesigning of the Crusader, the less obvious the financial advantage (I don't recall the numbers, but I believe the Spey Phantoms ended up costing over twice as much as the RAF buy when the project was amortized over the airframes). Redesign doesn't make much sense unless you get the USN onboard somehow, and that seems very unlikely. French participation helps, but they are looking at the financial aspects, too.