You ask me of the authorities drawn upon by Gregory in the collation of the Chronicles and how he came by the knowledge of such wondrous things from so diverse places and times.
Sicut Corvus Volat (“as the crow flies”) is a community made supplement for King Arthur Pendragon (6th edition) that details a wide range of settlements close to the normal player home base of Salisbury - which is the area I want to focus on - as well as a few other convenient tables for a GM wanting to add a bit of natural texture to the world.