tolls, the main bridge in Plymouth is just a few pounds
if you take any ferries (say in Cornwall) then they cost a few pounds
I don't think there is anything else along your route

Did you turn down the Exeter Station car rental for some reason?
If you want to be certain of automatic then hire an EV.

Here is a link to the TFL page for the congestion charge. https://tfl.gov.uk/modes/driving/con...on-this-page-0 . The charge is £18 per day so not a big deal in the greater scheme ... Unless you rent a diesel (v unlikely) the emission charge will NOT apply (rentals are not that old).

In terms of location, I would go for the cheapest and most convenient for getting out of town. As I may have mentioned previously, Marylebone is on the edge of the CZ and Sixt should be ok. It is a short run on the Bakerloo line, maybe 10-15 mins from Charing Cross which is a short walk from your apartment. Black cab or Uber would be slightly longer but may be better if you have lots of bags.
From Sixt it is a very short drive from from pick up to the A40 (Westway) which then continues on to the M40 . Once on the motorway it is a straight run via M4/M5 to the West Country. NB The 5 mile stretch from getting on to the A40 from Marylebone can get very congested. I do it most weeks it is usually ok in the mornings but it took me 30 mins on Saturday. Sixt should be able to tell you the best way to the Westway (it's only maybe 400 metres) avoiding the CZ.

On paper, the train may be more expensive, but slightly shorter on time (if no delays or cancellations . The drive is tedious and boring and could take much longer if you hit traffic problems - no way of knowing for certain.

Fron Exeter train station to the airport is 10-15 mis by taxi.

As suggested above, renting an EV is one way of ensuring an automatic (not that I think there would be a problem) but charging may be an issue. OK in London but you may have problems in Devon and Cornwall and you will almost certainly have to stop and charge en route to the West Country , especially in March.

Renting from Exeter Airport is doable but will just add time and bit of expense on to your journey. There seem to be Budget, Hertz and Enterprise depots in Exeter itself?

What is your date for travelling south west? If it's Friday 20th March I can see train fares to Truro for £42pp at 6am, 8am and 9am, Taking 4.5-5 hours. There is no way that you can beat that driving and if you do drive the driver won't be up for much when you get there.

I'm surprised that you can't fine an automatic car for hire. The car industry is rapidly changing here and now some 70% of new car sales are automatics. Given that's what car hire firms buy I would assume that each depot would have some now.

Hiring an EV would be a guaranteed way to get an automatic. I have been driving EVs for close to 8 years now and hiring one is my first choice almost anywhere I go now. Do you have any experience of them? It would be a perfectly fine solution, as long as you do a little planning when booking your accommodation. If you can book places either with a charging point on site or close by things will be very easy. For the longer journeys there are plenty of fast charging stations these days, but you want to only use them to top up because of their higher charging costs.

If you do decide to hire in London the simplest way to avoid the £18 Congestion Charge is to hire outside the zone, which will mean a short tube or taxi ride. I don't know the zone well enough to recommend somewhere but it should be straightforward. There is no way anyone will rent you a car not compliant with the wider ULEZ Zone.

We have never had or driven an EV car.

holiday is a great time try new things

I just did a quick request for a Hertz car picking up in Exeter and dropping off at Heathrow for a week from 20/3. I was given 12 options, 7 of which were automatic and two were electric. All between £275 and £320 for 8 days.

There is nothing to be feared in choosing an electric car.

John, are you in the UK? Earlier I tried to rent a car with budget at terminal five. We had done that last year and it was very easy to drive out and then we just stayed at the Holiday Inn where we dropped the car off. I was not able to find that same location again. Today I realized that I needed to go to budget.co.uk/rental. Then I was able to find it. I am curious if you are using a UK site to come up with the Hertz rental. I did just find what you were talking about, but I did need to go onto a UK site. I was not doing a UK site previously so many of these locations were not coming up, I find that odd. Some of the smaller towns also do not do one-way drops.

Yes I am in the UK and I was using the Hertz UK website. I also looked at Budget's UK site just now but that only showed cars at Exeter Airport and not in the city and in any event was much more expensive than Hertz. Thrifty also seem to be in Exeter as well.

autoeurope show all of these sites and you can compare all their prices on one screen

We ended renting both ways just outside of Heathrow, Bath Rd. It is with Budget and at the Holiday Inn location. We rented from them at the same location last year and it worked well and then when we returned it we stayed at the Holiday Inn, cheaper than the Sofitel and easy! Hopefully traffic won't be as bad leaving London on a Friday morning. I must say that I became a little anxious when someone mentioned train strikes. We were stuck a year and a half ago on one of the small islands in the Azores for three days because SATA went on strike. I was a day and a half late to start my Camino trek because of it. I do not hear of the UK doing the strikes as much.
We will probably arrange for 2 cars to take the 10 of us to the airport, the family will catch their flights and we will pick up the car and drive to Dartmoor for the night. Then I think we will have 4 nights in St Ives, 3-4 nights in Padstow and possibly go through Exmoor on the way to Heathrow. Any hotel recommendations in any of those locations will be appreciated.
I am grateful to each of you for all the help that you have given me.

Post #2 above, hotel with parking at the front door, St. Ives.

I have written Thurlestone House because it looks as though they may be closed in March.

I learned something interesting today and thought some of you would be interested because you are all so knowledgeable and helpful. Several of the suggestions that were made to me on this post I was unable to find because I was searching on a US website. So when I put in the UK site (for example Budget) I was able to see which rental locations and rentals you were seeing. I noticed on the UK rental sites I was not able to exclude the CDW & LDW coverage because all UK sites include that coverage and require it. When we rent a car, we decline the CDW & LDW coverage because it is included on a credit card that I have or it negates my coverage with the CC. So, if I want it excluded than I was told I need to rent on a US site. Then many of those car locations, for example, the one that we chose outside of Heathrow is not on Budget's US site. I hope I was able to explain this clearly are not available.