Hi Anne -
At the risk of being accused of bias, I would agree that trying to fit both the Yorkshire Dales and Cornwall into the same trip in the time you have is spreading yourself much too thin and is unlikely to do much justice to either region. If you have decided the Dales are a must, then stick with that (bias now on full display). As a ‘native’ if there’s anything you think I might be able to help with or advise on, please feel free to ask, either here or by PM.
One final thing. If your flight home is late afternoon from Heathrow, I would definitely want to be at Heathrow the evening before the day of departure, and stay overnight. To attempt a trip from the north on the same day is too much like flying by the seat of your pants for someone as risk averse as me. Plus you may still be having to factor in the continuing rail strikes, so I would ease any tension that attempting same day as departure travel might incur by allowing yourself a little ‘wriggle room’!
Other than that, have a great trip!
Ian
P.S. Book your rail tickets in advance. If you buy as ‘walk ups’ on the day, they’ll have your eyes……