Interesting concept. I like what you are doing, but if I can offer a couple of suggestions? Personally, I don't like Francia, Old Castile and New Castile as names for Duchies. Perhaps you can use Valois for Francia, Toledo for New Castile and possibly Burgos or Soria for Old Castile. As for the Kingdoms, I wonder why you dropped the Kingdom of Navarre? Also, and this may just be my preference, but I have always like the title King of the Occitans instead of King of Aquitania. Just some thoughts I have, but I do have to say that this is a very nice idea, and I definitely like how you combined Brittany and Wales into a Kingdom of Britain (again possibly calling it the Kingdom of the Bretons?)
Cheers,
~Hawk