Train to Cornwall area ideas

The largest cities in that area are either Exeter or Plymouth - the former is further away from Cornwall but much more attractive. Or there is a Europcar branch right at the end of the railway line in Penzance.

If you are flying into London, presumably Heathrow or Gatwick, you won't actually be in London. It's actaully easier to pick up a car from the airport as it's a short and easy drive onto the motorway, particularly Heathrow. That way you can take your time to adjust to driving on the other side and getting used to the car without having to worry about city or town driving. You'll also have a larger selection of vehicles to choose from, particularly automatics if that's your requirement. You also won't have to pay a fee to drop it off at another location if you intend to return the car prior to your flight.

Two weeks ago,we traveled by train from London to Plymouth, rented a car and spent the weekend touring Cornwall before returning the car and taking the train north. We wanted to make our stop Exeter, but were not able to arrange a car rental in Exeter with a Sunday return. You might check out those details as you set up your rental arrangements. We ended up renting from Enterprise.

Thanks for ideas, Exeter, Plymouth or Penzance, but check for availability and opening times.
I often do rent the car at major airport and drive away, Lisbon, Rome and Paris as recent examples. But with perhaps about 4 hour drive each way to Cornwall and back and no real plans to stop in between, I thought train would be alternative.

A lot of people here think it's a very bad idea to do a long drive immediately after a transcontinental flight, due to the effects of jetlag.