It's not just airframes there was a whole political angle to the UK F4 buy...which started with,if not before, the P1154, which in turn led to the ill fated F111K order which in turn led to the F4K/M purchase.
Add to that as has been said the Buccs becoming available, and the Jag on it's way and you can see why the F4 ended up as a full time ADer....