AVOID AVOID AVOID the A303: you'll lose the will to live as you trundle between dual and single carriageway, getting stuck behind 98 year olds driving at 30mph, tractors, lorries and the legendary tailback along Stonehenge. You won't see any scenery because you'll eyes will be puffed raw from weeping in frustration. Completely recommend M4 to Bristol, then M5 to Exeter. If you are going South (i.e. Liskeard / Looe) don't go A30, but A38 through Plymouth and over the bridge into Cornwall. Edited to add: Vauxhall, Earls Court, Hammersmith, Chiswick, M4 way out of London. Enjoy.