Transportation options and Cities/towns for a base in southwest England

I can't help with info on public transportation in Cornwall, we always had a car. Poldark was filmed in ten or more locations in Cornwall, so you might want to decide which ones you want to see. I can highly recommend going north to Holmfirth, where Last of the Summer Wine was made. We spent two nights there and loved it.

Look at Weymouth for Dorset - there's a train station but you'll also need buses to get to parts of the Jurassic Coast. Lyme Regis is another option.

https://www.visit-dorset.com/

For London stay in London - commuting from elsewhere is false economy both in time and sanity. If you're determined to stay outside London look at St Albans.

I agree with royaume-uni about staying in London rather than commuting in and out. Winchester is a good place to visit, as is Salisbury, but both too far from London for practical touristic commuting. Even on a 4-5 week trip (lucky you!), you don't want to be riding trains every day in and out of the city.

Oxford and Bath would be good base cities, with buses and/or one-day tours to nearby places like the Cotswolds, Blenheim, Stonehenge, and Avebury. Bath also has good train connections to Bristol and I believe Cardiff.

We did a driving tour of Cornwall (someone else drove, we paid), basing in Penzance. There are buses but I don't think they would be a very convenient way to see many places. This is one area, like north Wales or the Cotswolds, where driving is really the best way to sightsee.

For info on train connections, use National Rail. You can also buy tickets on it, or at stations. Advance purchase saves money but locks you into a particular train. We took a fast train from Reading to Exeter (it originated in London), and on to Penzance with many stops. But a train will give you only fleeting glimpses of Cornwall's coastal scenery. There are many fast trains each day between London and Oxford or Bath.

For the Jurassic coast (Broadchurch) I recommend Lyme Regis. Lovely little town. Weymouth is a big town, slightly far from the Jurassic coast, Lyme Regis is on it
You will need a car to explore this area.

"I wouldn’t mind doing this for London as well - finding a city to commute from - Winchester has been mentioned."
Just curious...who suggested that you use Winchester as a base to go back and forth each day to see London? If that's what you are saying. ???

Winchester is 67.8 miles from London. Too far away to use it the way you had wanted.

If you want to see London, stay in London. I don't know how many days you want to spend there, or what sights you want to see.
But if your budget has you avoiding the high London hotel prices, we (on this Forum) can help you find a reasonably-priced or budget place to stay in London.

London is also a good hub to go out from on day trips, because the train lines go everywhere from London.
A good company doing day trips out from London is the London Walks Company. They do day trips to many, many places. Oxford, Cambridge, the Cotswolds, Salisbury, Stonehenge, and many more places. Inexpensive. You go out early morning on the train with a group leader, then back by train at the end of the day. www.walks.com
They also do excellent walks around London that last one to two hours.

Do you have a really good large map of England that you are using to plan this out? This is essential. A detailed map is the Michelin driving map. Even though you will not be driving, you will need a detailed map.

You will want to buy a copy of the Rick Steves England guidebook, if you do not have it. It covers a lot of rail and bus information. It also gives places to stay and to eat in some of the locations you've mentioned. "Possible names of base cities/towns for day trip exploring - walking/hiking as well as historic sites?" The RS guidebook will help with this, too.

Many places in England have good bus service going out into the countryside where trains do not go. Sorry; I am not familiar with the bus routes in Poldark country. Maybe someone here on the Forum will chime in with some help on that.

"Use some kind of train pass"--Read here about the Great Britain rail passes and train travel:
https://www.ricksteves.com/travel-tips/transportation/trains/britain-rail-passes

You could also do a tour with these people, Brit Movie Tours. I have planned to do a tour with them in May. Here is their Poldark Tour, making your trek a good bit easier. It looks fab.
http://britmovietours.com/bookings/poldark-tour-of-locations/

From the Visit Cornwall website, a map of Poldark locations:
https://www.visitcornwall.com/poldark/blog/poldark-film-locations#
And a tour based in Cornwall, with some accommodation suggestions on the right side of the page:
https://www.visitcornwall.com/things-to-do/activities/film-and-literature/poldark/poldarks-cornwall
https://www.visitcornwall.com/poldark/blog/5-to-try-poldark-experiences

Walking tour:
http://www.walkitcornwall.co.uk/en/

Thank you for all the responses - I did wonder if not driving would be a problem - this was why I posted the question. Obviously visiting London from a base outside the city does not sit well with a few of you responders - I appreciate that tip. I think the idea of finding escorted tours from London or possibly in the areas I am visiting is the best option. I can search online those places in the areas I hope to visit - thank you for the Dorset link and specifics as in Lyme vs. Weymouth. I look forward to possible help with London accommodation on here when I post that question. Yes Rebecca I will be buying Rick’s book and no Rebecca I do not have a detailed England map - yet. Thanks for the many links in your response that I can check out. I planned to go to the online train sites. This trip is hopefully happening in late June and early July so I am just in the early planning stage right now.

"I did wonder if not driving would be a problem - this was why I posted the question."
"My question is how easy will it be to reach these places via trains preferably - I will not be driving myself or will I have to rely on buses too?"

To address some of the places you mentioned.

***Oxford--Easily reached from London by train. You can go from Oxford to a great many places by train.

***Cornwall--Train goes from London. Stops include Bodmin Parkway, Falmouth and Truro, among others. Look at this rail map for other stops you would need in Poldark country.http://www.nationalrail.co.uk/static/documents/content/OfficialNationalRailmaplarge.pdf

Using the Poldark sights map I gave you in my last post, you will want to use one of the towns on this map as a base.
https://www.visitcornwall.com/poldark/blog/poldark-film-locations#

Here is a bus map of Cornwall. It has the zoom feature, so zoom in on the areas you are interested in.http://www.cornwallpublictransport.info/images/countymap.pdf
Unfortunately, it would probably be easier for you to get from one location to the next (touring Poldark sites) with a car. Or...use the tour I gave you.

***Bath--Easy to reach by train from London. Good connections for leaving Bath and going to many other places by rail.

***Stonehenge--Not close to Bath. It is close to Salisbury.

***Salisbury--Easy trip from London by rail. Good rail connections for leaving Salisbury and traveling to other places.

How to get to Stonehenge without using a car.
Method #1--Take the train to Salisbury. From the rail station OR the downtown square near the Tourist Information Office (either location will work). Take the Stonehenge Bus. It is painted with the large words "Stonehenge" on the outside of the bus. You buy a ticket from the bus driver which includes the bus ride out, admission to Stonehenge, and bus ride back.

Method #2--If you are in Bath, contact Mad Max Tours. They run a small mini-bus to Stonehenge and several other sights.
www.madmaxtours.co.uk

***Dorset - Jurassic coast--Don't have any information. Never been there. Someone else will have to answer.

Hopefully our train expert will come along and add his thoughts on your question.

"or will I have to rely on buses too?"--Yes, in some places out in the country, you will take a train to the station nearest to the town you are wanting to get to. Then take a bus the rest of the way. Since you have not specifically named these small towns yet on your itinerary, no one can give you specific advice yet which buses to take. When you have a specific, day-by-day plan, if you come back here and post that, people can give you advice on what bus to take from one town to another.

"I did wonder if not driving would be a problem - this was why I posted the question."
In some areas, you are covered thoroughly with train lines.
However, in some areas--Cornwall--you really could use a car once you get there.
Not driving will pose a problem, but only in Cornwall.

I hope I have answered some of the things you didn't feel got answered by the first batch of answers.

Good luck planning your trip, and happy travels, Judy! I hope you have a great trip. Sounds like a lot of fun.