That is possibly, going to be very hard to get as most companies won't offer critical illness to type 1 diabetics. One of the last companies to offer such cover was Legal & General, but they no longer do. Not so sure for type 2's but that was always my bug bear, they insure Joe Bloggs next door, for a small amount, but load polices for diabetics.
I can't imagine that there would be many people who get a pay out solely because of their diabetes. Yes, it's a lifelong disease and, if not managed, can have a serious impact on your life. But, it is manageable and, for most people at least, does not stop you living your life as normal.