Nobodyman these might be useful to you. Just from googling, here are:

Sites within Cornwall from myth:
[url=http://www.king-arthur.co.uk/_/rsrc/1260977249634/home/Arthurian%20Map%201%2Btitle%20gs1_edited-1.jpg]

The general layout of countries in Authurian legend:
[url=http://www.earlybritishkingdoms.com/arthur/images/literarymap.gif]
or
[url=http://www.avalon.co.uk/map.gif]

Lyonesse, where Tristan is from, is a sort of Atlantis of British mythology which sank into the sea. So imagine the peninsula that is Cornwall extending a bit further with the small kingship on its end, a defense against raiders from southern end of the channel (normans/franks,mostly. What we now as France today if I recall correctly.)

Its up to Nobodyman if we want to use these officially.