We are taking train from London to penzance, then renting a car in penzance for a week, then dropping off in Exeter. Any tips other than renting an automatic transmission! Looks like dropping off in Exeter isn't that close to train station-is that right?
In Devon, Dorset and Cornwall the driving can be very rural, even minutes outside large towns. Given the absolutely miniature size of some of the lanes, and the fact that even though they are only a foot wider than each edge of your car - they are for both directions and sometimes a lorry, I'd suggest a smaller car with good all around visibility and one that you will be comfortable reversing in. I suggest you become familiar with the driving style that I refer to as "Dynamic reallocation of lane-width" which means learning how to share the road and work together to get everybody to their destination safely if not promptly. You should know that it is your responsibility to be aware of passing places and that the car nearest one either goes into it, if on their side, or stops opposite if on the other side, or reverses to a passing place. Big trucks always win, and farm machinery always wins. So do 18 year old males determined to make a mark in the world. You need to be familiar with the UK pictograph road signs, and should memorize national speed limits - 70 on dual carriageways and motorways (don't worry about motorways in Cornwall - there aren't any, and just the occasional dual carriageway) 30 in built-up areas where there are streetlights (the streetlights may be switched off due to austerity but still indicate a built-up area), and 60 everywhere else. Unless over-ridden by speed limit signs which always take precedence. Always pass or overtake horses wide and at walking speed. Flashing yellow lights on farm machinery mean slow speed - under 20 mph. Don't stop at give-way signs unless you need to. Many accidents are caused by the second driver expecting the first to act normally. Dynamic reallocation of lane width also applies in towns with cars parked on the sides of the roads.
Herds of sheep or cattle have the right of way. If the road/lane you are on goes through a gate close it behind you. Shoulders on roads are very rare. Expect sharp kerbs, often of stone, and bridges that come right to the edge of the car. Have you driven in England before? Devon, Dorset and Cornwall are like the rest of England only much more so.
Nigel gave you an excellent summary of the rules. I'll only add that it is considered poor form to honk when approaching a blind turn on a narrow, one lane road. I must admit that my nerves have gotten the better of me more than once, though. Also, beware the speed cameras which seem to be everywhere. A GPS unit or smartphone app can alert you to the fixed cameras, but there are mobile cameras which move around. Probably not a problem on narrow twisty roads, except going through villages.
Nigel, you took me right back to my driving adventures a couple of years ago! One of those stone kerbs took my left front tire and only the great goodwill of a bystander and some garage guys in Wrexham kept our trip on schedule. Marylinne, I would recommend an auto transmission to save you having to learn other-sided shifting along with all the rest, but it may also mean a bigger car which you won't appreciate on those country lanes. We haven't been to Cornwall but we want to go as soon as I'm ready to take on the driving again (aging doesn't help). I don't think there's a satisfactory alternative to driving in that part of England, so good luck, take your time, and keep smiling!