Dartmoor

Presuming you are driving there Bristol Airport is a lot closer, and much easier to get to and from.
But that in a way depends on the rest of your itinerary, as you are presumably going to other places apart from Dartmoor.
Exeter Airport is even closer, but that only sees domestic flights from regional airports like Belfast City, Edinburgh and Newcastle.
Public transit is quite thin on the moor.

If you are lacking a car then Wistman's Wood may more difficult to reach than it's worth. Though it's an exceptional sight there are places similar all through Dartmoor.
From London, Okehampton (via Exeter) or Ivybridge are accessible by train. Moretonhampstead, Buckfastleigh and others within Dartmoor are reachable by bus (also via Exeter). You can get to Bovey Tracey, on the eastern border of the moors by bus.
Are you planning to stay close to Dartmoor? It's quite the journey to come from London for the day.

Thank you for replying. The woods are one of the highlights of our proposed trip. We haven't determined where we would like to visit as this is a repeat destination. We saw quite a bit of England and a little bit of the Welsh interior in 2016. We are not opposed to hiring local guides and we do not plan on renting a vehicle.

There is literally one bus a day- the 98-https://bustimes.org/services/98-tavistock-yelverton-2
Both Tavistock and Yelverton are on the frequent #1 route from Plymouth. If you wanted to make the morning bus you leave Plymouth at 0820.
It is a good 3 miles walk from Princetown, where the marginally more frequent bus turns round, but it would be a tight timed round trip walk even then.
All in all, if you are determined to do this without a car I would say get the afternoon bus, stay at the Two Bridges Hotel, then catch the morning bus out next day.
Or stay in Plymouth the night before, get the morning bus out/afternoon one back and you will be back at Plymouth Royal Parade at 1511.