Surely, you mean the south-west off England as shown on this map? https://www.google.co.uk/maps/@50.4186151,-4.503728,9.76z/data=!5m1!1e1

We are planning a trip to Devon and Cornwall and I posted a couple of threads with questions. Here is what I saved.
CORNWALL
https://www.rabbies.com/en/england-tours/from-london/5-8-day-tours/devon-cornwall-a-magical-land-of-legends-5-day-tour
https://www.viator.com/tours/London/5-Day-Devon-and-Cornwall-Small-Group-Tour-from-London/d737-2250LON4
I can recommend the Cider House B&B in the grounds of Buckland Abbey. http://www.cider-house.co.uk/index.php
This is a beautiful area of the Tavey Valley, you'll pass Dartmoor ponies and it's not crowded. Buckland Abbey was the home of Sir Francis Drake and you'll get complimentary tickets to tour the house (it's a National Trust property). After 28 days last year I think you'll cope just fine!
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This may be more than you are looking for but I've done the Road Scholar Cornwall program. It says 10 days but they count the day you leave the US as Day 1 and of course count the departure day as Day 10. You meet up in London for the first night, ride out to Cornwall in a coach stopping in Exeter for lunch and a tour of the cathedral, then are based in Falmouth for 6 nights, return to London for a night at Heathrow as the last night of the trip.
https://www.roadscholar.org/find-an-adventure/22431/Cornwall-England-s-Hidden-Treasure
The long distance ride is in a full-sized coach. The day trips from Falmouth are in a smaller 18-person van so you can get down the skinny lanes and back to cool things.
On the Road Scholar programs, like the Rick Steves tours, I arrive in time to have independent time on my own in the arrival city, so for this one I spent about 10 nights ahead in London at a different hotel then moved over to the Road Scholar tour hotel.
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https://www.rabbies.com/en/england-tours/from-london/5-8-day-tours/devon-cornwall-a-magical-land-of-legends-5-day-tour

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Look at the following map - the areas coloured green are the most scenic - but you would need to drive yourself to see most of them.https://www.google.co.uk/maps/@50.6914159,-3.9433723,9.21z/data=!5m1!1e1
These places are very attractive - find them on the map and then Google them. DARTMOUTH, SALCOMBE, POLPERRO, FOWEY, EDEN PROJECT, LOST GARDENS OF HELIGAN, TREBAH & GLENDURGAN GARDENS (near Falmouth), ST. MICHAELS MOUNT, MOUSEHOLE, PORTHCURNO (Minack Theatre - curt into the cliffs), LAND’S END (although commercialised), ST.IVES, PADSTOW, TINTAGEL, CLOVELLY, LYNMOUTH, SELWORTHY, DUNSTER. Avoid Newquay! Truro is fairy attractive and being central, makes a good base for touring Cornwall.
Roads across Dartmoor can be ‘challenging' for foreigners. Exeter is worth a look but can be difficult to drive in and out of due to congestion. Plymouth has the quay from which The Pilgrim Fathers set sail for America but the city is not that interesting/attractive.

Regarding driving or not - zoom down on the map and then select street view for some of the places mentioned. Most have tiny narrow streets and as a result, car parks are found on the edge of towns.

Look at the following map - the areas coloured green are the most scenic - but you would need to drive yourself to see most of them.https://www.google.co.uk/maps/@50.6914159,-3.9433723,9.21z/data=!5m1!1e1
These places are very attractive - find them on the map and then Google them. DARTMOUTH, SALCOMBE, POLPERRO, FOWEY, EDEN PROJECT, LOST GARDENS OF HELIGAN, TREBAH & GLENDURGAN GARDENS (near Falmouth), ST. MICHAELS MOUNT, MOUSEHOLE, PORTHCURNO (Minack Theatre - curt into the cliffs), LAND’S END (although commercialised), ST.IVES, PADSTOW, TINTAGEL, CLOVELLY, LYNMOUTH, SELWORTHY, DUNSTER. Avoid Newquay! Truro is fairy attractive and being central, makes a good base for touring Cornwall.

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Devon & Cornwall remain pretty much the most popular destination for British holidaymakers staying in the U.K. Roads down to the SW in the summer are notoriously clogged with traffic.
From Heathrow, either take the Heathrow Express to Paddington and take the train to Exeter or the bus from Heathrow to Reading and pick up the train there.

Heathrow hast its own rail station. Actually different stations for each terminal The trains only go into London, but you have a choice of the tube or a dedicated train to London Paddington. In your case the latter is better as trains to Devon and Cornwall also use Paddington station.

Cornwall with a car is a necessity for me . I like the idea of taking the train as far as Exeter and then hiring the car , On the trips that I have made to Cornwall , I picked up the car at Heathrow and drove , but I made a number of stops along the way , thus breaking up the drive . There is a lot to see , you need to read up and make your choices accordingly . The Tin mining history and the impact on the Cornish economy was fascinating . As well , the small village of Coverack on The Lizard was a charming place . This book , by Mark Kurlansky has a fair amount of information about the fishing industry in Cornwall ., and its parallel to that in Gloucester , Massachusetts - https://www.amazon.com/Last-Fish-Tale-Atlantic-Gloucester-ebook/dp/B0017T0A30/ref=sr_1_1?crid=2041EN8VJ7NPE&keywords=the+last+fish+tale&qid=1562446637&s=gateway&sprefix=the+last+fish%2Caps%2C142&sr=8-1
I spent a week in Cornwall with a vehicle and did just fine. I honestly can’t imagine getting to see Cornwall without it. Sure, there are a lot of narrow roads, but having the ability to see the countryside and stop where I wanted was great. What I wanted to see was very spread out, and Cornwall’s public transportation does not get you everywhere. But I guess it depends on what you want to see...
I really enjoyed Tintagel, Minack Theatre, Mousehole, and St Michaels mount. St Ives is a popular destination, however I preferred the smaller, less touristy areas.

Hi from Wisconsin,
We spent a week in St. Just down near the tip of Cornwall, near Land's End. I had been there in 1973. My wife had always wanted to go there and then two other things helped push us. Doc Martin's Port Issac. If you go to "his village" you will be pleasantly shocked. It looks just as it does on TV. And then there is the shirtless Poldark. I mean the lovely coastal scenes in the Poldark series. And St. Just is where a heck of a lot of it is filmed.

There are nice walks along the coast, megalithic stone circles and standing stones. Mousehole. Falmouth has a castle. St. Ives is crowded to the max. So different than St. Just. Actually you will be lucky to find a place to rent near St. Just.

I think it is Paddington station that provides a train all the way to Truro. I would check car rentals along the way. It might be that Exeter has the lower rental prices. Penzance has a Europcar rental at the train station. They are a solid company. Wen you rent a any car take photos of All sides of the car. And include the bottom side of front and back. With a digital camera there will be a record of when you took the images. I also take a picture of the odometer. And when returning the car do the same.. Keep the petro refueling slip and take a foto of that. Other wise a rental could end in she said, he said.

Don't spend too much time driving.

wayne iNWI