The Kingdom of Cornubia is based only in the modern day area of Cornwall, England at the beginning of the game.
In game, the Kingdom of Cornubia is listed as a Christian kingdom, and all Cornish settlements are Christian. In reality, Cornwall was the last area of England to remain Pagan, and at the time the game is set would likely have still been Pagan in some areas further from large populations.
The Kingdom of Cornubia is one of the easiest factions for the player to defeat and capture settlements from, as it is weak to begin with and is usually further weakened by wars with the Kingdom of West Seaxe.
Wikipedia has an article on this subject at: Cornwall
