Easy, Cornwall has 2 major roads. One on the Atlantic side of the peninsular, the other is well, the channel side. Channel highway sounds a bit naff. And A30 and A38 get a bit dull. Esp considering the A38 can be called many things as it runs in an interesting meander right up to Derby and beyond.
I'm considering riding down the southwest on the old A38, there is a cheap campsite on Atlantic Farm (guess where that is) and it would probably take a day or two. Running through Worcester and Bristol, Bridgewater, Taunton, Exciter etc, seen how many times the junctions of the M5 lead on to the A38?