I recently stayed at Cley Windmill, North Norfolk. Having driven / walked past it a million times, it was nice to see the inside. It really is a unique place and I can recommend the round rooms for a stay. The highlight was looking out over the reeds in the morning and evenings, to see Barn Owls and Marsh Harriers flying past at regular intervals.
I can also recommend the restaurant, where a set menu is served each evening (and is open to non-residents).
If you get the chance to stay there, do it, and make sure you get out onto the marshes at dawn!