There is not even a need for marches to defeat the Iberians powers once you have seized a couple of islands (canarias, baleares) :no navy in game is a match for the Royal Navy, even without naval ideas. (With naval ideas, it's an exercice on how quickly you can sink their fleets)
I'm just thinking about vassals I can pretty much leave to fight the continental wars. Obviously I could drop off my troops but this isn't the British way. The British way is to stand off the coast and watch other people fight. Then once victory is assured you land in force and dictate terms to all the exhausted parties.