It would be a real pig to shoot down. The Mavis came with 5 20mm cannons. Talk about a porcupine. Of course, perhaps it should be countered with the Short Sunderland, and perhaps some dissappearing submarines? Subs would be cool to hunt those nasty CVs and convoys.
Oh Oh, and how about different sizes of CVs, like the CVE of the Americans. It would be smaller, faster and able to handle a small number of aircraft.