From Wales, you'd need to head back across the Severn Bridge on the M4 Motorway, then take the M5 south at Almondsbury Interchange (Junction 20) and follow this all the way to Exeter.
The M5 ends at Exeter at junction 31 and automatically becomes the A38 - follow this for 3 miles until you arrive at a split where you then have a choice.
You can either stay on the A38 which will take you down through the southern fringes of Dartmoor to Plymouth. if you choose this route, the A38 bypasses Plymouth and will bring you out on the Tamar Bridge where you cross into Saltash, the first town in Cornwall. You then follow the A38 down into Cornwall via Liskeard to Bodmin where the road ends.
After the M5 ends at Exeter, folow for three miles on the A30 then take the A30 westbound which is signposted for Okehampton and Bodmin. In my opinion, a much more scenic route that follows the northern edge of Dartmoor, and bypasses Okehampton and Launceston before arriving at Bodmin, as does the A38 as per the first choice of routes above.
To get back to Bristol Airport, take the A30 out of Cornwall, follow all the way back to Exeter, then pick up the M5 Motorway and follow to Junction 21 south of Bristol from where the airport is signposted.
If you would like any specific routes whilst actually touring Cornwall, please let me know.
If I were driving, I'd go across the bridge as suggested (unless I could find anothe route - I hate bridges) then head south toward Taunton. (In fact, I would go north in SE Wales to Abergavenny or Wrexham). There are places in Taunton to stop or go down to Bodmin. Again, there are places there in all price ranges from inns to expensive country houses.
How far are you going into Cornwall? There is a lot to see and do. Penzance and a helicopter ride to the Isles of Scilly and/or Lizard Peninsula are both on the tip. St. Ives on the Atlantic coast. Redruth towards the middle. Falmouth on the Channel has a ton of attractions. Although I'll be at the records office in Truro, I'll be based in Falmouth for a week - Pendennis Castle, St. Mawes Castle, and the first sights mentions. Then on to St. Austell and the Lost Gardens of Heligan - lots of B&B's there - try Allen's. Up to Liskeard - various things to do - Elnor Guest House your best bet. Launceston - lots to do. Plymouth - a large town you probably don't want to drive in, but much to do and places to stay. Probably more expensive, so I would stay outside of town. Then head to Exeter. Lots to do and places to stay. You can go on directly to Bristol, but there is much to do in between including Salisbury, Stonehenge, Bath, and surrounding areas. Part of this is in the Cotswolds. You may want to spend time with a MultiMap and the internet. TripAdvisor has a lot of hotels & B&Bs rated; I use it a lot.. I tend to stay in those less expensive so have reviewed those. 2008 will only be my second trip, but I've planned both itineraries. I'm always working out details on 2008 and just added an extra day so I can spend more time in Launceston. In fact, came on the Forum to find accommodatons there. Previously, I'd added an extra day to go to the Lost Gardens of Heligen. Also accessilbe from St. Austell is the Eden Project which is supposed to be very interesting. It's all or mostly enclosed in large bubbles near a former mining area. There are also some former mines open to the public. Depends on what you want to do, what you want to see, where you want to go, how much time you have, and the amount of money you can spend (average 50 GBP/night). Unfortunately, the last two items seem to dictate it all. I have limited resources and time, so am cramming in a lot of things and places.
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