umm - Heathrow. That's if I've read the question the right way round
There's nowhere in the southwest that has domestic flights from Heathrow
London City may be getting flights to Exeter later in the yeatEdited: 14 September 2014, 16:52
Don't think there's anywhere convenient you can fly to from Heathrow.
If you're going direct from Heathrow to Cornwall then airlink coach to Reading and then train to somewhere you can spend a night and then hire a car is probably your best bet. If you intend to stay in London first, then you can get the train from Paddington or hire a car and drive all the way.
In 2012 we flew into Heathrow and took the train from Paddington to Penzance, as has been suggested above. Took a taxi from Penzance to our cottage in Paul (just up the lane from Mousehole), then collected our rent car in Penzance the following day. That's what I'd do again, instead of driving all the way down from the London area.