if Devon or Cornwall is your preferred choice debbie, stay in devon its central to all that area, hire a car there or use public transport. You will find it easier to travel to london by rail or cheaper by bus, this way you can totaly relax and leave your luggage at your hotel/B&B'Cottage/Caravan/Mobile home. (a mobile home is a large static 'trailor' on a holiday 'trailor' park. These are a good place to stay if your are sightseeing in a rented car. they are well equipped, inexpensive and a great way of getting to know other people. plus you can eat when and what you like. Theres lots to do and see in devon and its right next to the county of Cornwall. south Wales is just a couple of hours drive away over the 'bridge'. Heres what I would do if you have decided you are going to that area.
Fly into the uk. train to Devon destination. Hire car. Hire trailor (Mobile Home) on selected park. leave luggage in 'trailor' whilst traveling around daily then going back to 'trailor in the eves. Most Parks have a Bar with entertainment provided, ideal to meet people over a glass of beer/wine. They will usually suguest places to visit. Go up to London for a couple of days, staying at a B&B (Leave luggage at 'Trailor Park' travel light. The cotswolds are also easy to drive to from Devon. So to summrize, you have the North and south coast beaches and the roling hills and Cheddar George, Whookey Hole etc of Devon. then the rugged landscape of Cornwall close-by, or the nearby South Wales. Another day a drive to the Cotswold villages. another couple of days up to London.
hope this has helped but I'm dissapointed you havnt come to the North of England, maybe some day lol Enjoy your visit.