Jacko,
You miss the point. the only reason the military do anything is because the politicians want us too. The US may not have needed us to achieve the military aim but the sure as heck found us useful from the political angle and the manifestation of it was the forces you mention and as ILLUSTRIOUS was in the region the politicians directed its participation. As I said earlier this is exactly why the politicians want CVF/JSFand are unlikely to allow its cancellation.
All Diego Garcia can deliver is more bombers (and only the big ones).