Airplanes can attack ships. Destroyer is needed if you want to attack Subs.
As for invading England it is possible. I have done it before. Those first 12 IPCs you get, can easily go for a destroyer for the Germans. Following that. You need to take the convoys in the Atlantic. You should have a good amount of land units as it already in the game. I suggest just mainly buying transports with perhaps a destroyer here and there depending what the UK does. Besides Destroyers give you that nice bombardment which according to the rules I have is 3 or less. (I think in 2nd edition rules it is “2” or less). Either way when you know you have more forces then what is currently on the UK, send your airforce and units in. In the mean time before that just keep sending units accross the channel to keep the UK forces down.
In short you have to be very careful. Usally if you cannot take UK quick enough the USSR will take you over. For me it comes down to being 1 turn for victory or defeat.