Hi Jon,
If it is Cornwall you are interested in, you may wish to contact Bill Jackson as he has organised 2 CM rallies there, and he might be willing to share the road books. Some great roads, although I remember we went ‘off piste’ (deliberately), and almost got stuck in a very narrow lane with granite poking out through the undergrowth!
Picture attached for amusement value!
We ran a CM in Dorset a few years ago, and can provide some great driving roads there. Dorset may be a little too far east for you though…..