It's all motorway if you're travelling by car. Take the M6, at around Birmingham pick up signs for the M5 (south west and Wales) and eventually you'll reach the A38 which will take you straight to Plymouth
Tried & tested. My wife & I looked at the map. We did the trip the other day & had an excellent time & journey. M1 down to the M25 (have a look around London) or stay on round to the M4 over to Cardiff (have a look around) onto the A38 Plymouth in no time.