The main A30 through Cornwall & Devon (or Devon & Cornwall for JOGLE) is notorious - much of it is motorway in all but name yet cyclists may ride on it. 

MickF from the CTC Forum has kindly created both .gpx and .pdf map files for downloading (free of charge) - see the link under Resources/Planning

The content has suggestions for a route from Land's End as far as Tiverton Parkway, with a supplementary route avoiding the A30.

Indicative map