Assuming we are not counting the Channel Islands, I would say, Wight, Angelsey, Man (does that count as English?), Sheppey and a toss up between Lundy and St. Marys in the Scillys.
The Isle of Wight is No.1, followed I believe by the Isle of Sheppey . I believe that Foulness Isle is no.4 but I can't think of No.3. No5 may be Canvey Island but not sure. Stew.
Could this be a trick question? there are several Isles that are IN England(as per the question) rather than off it i.e. Isle of Dogs, Isle of Axholme, Isle of Grain, Isle of Purbeck, Isle of thanet;