We could always say that a lot of things that were supposed to happen to create England did not, and then what ensued was a division of the land into small county-states, with each county having its own king. Better yet, as the game starts out, we can say all the areas but area XYZ got wiped due to some disease spreading like wildfire - and that makes it possible to do an almost clean start, while keeping some medieval concepts and RL-History-ish restriction and allowing to truly let imagination loose to create a realistic world. It also takes care of those who think "oh, it's a made up world, so let's do magic!" - since this is supposed to be simply an alternative story to RL. I wouldn't mind using RL maps to restrict possibilities - though I suppose natural resources' location could always be tweaked somewhat - I'm not looking for de facto recreation of England, just a likeness enough
