It isn't policy to do any of these things autonomously, without allied support. If it was then we'd need adequate SEAD of our own and an F-15 class air superiority fighter every time we deploy a GR4, a Harrier or a Jag. Instead we rely on the Spams or the Dutch or whoever for fighters, and on the Germans and Spams for SEAD. It's not ideal, it 'goes against the grain' but it works.
In just the same way, in those rare circumstances where we need Fleet Air Defence, we'll expect to get it from our Allies. Not an ideal solution, but it's only a capability gap, pending introduction of FCBA, anyway. And if we had to get rid of one frontline fast jet type, Sea Harrier was the one to lose. It's a shame, and we'd be better off keeping it, but that's not an option, financially.
Simple as that.