Next up? Wye Valley, Shropshire, Herefordshire, parts of Wales

I would be looking at flying into Bristol Airport on Aer Lingus via Dublin, although Birmingham via Dublin is an easy alternative which could work. Then make your way south to north.
You could open jaw back from Manchester, or from North Wales Birmingham would also work well.
Trains off the Cambrian line go direct to Birmingham Airport.
From Chester and the North Wales coast there are also direct trains to Birmingham New Street (change for the airport).
Porthmadog has two steam railways (technically 3)- the Welsh Highland, the Ffestiniog and the Welsh Highland Heritage (the latter long pre dates the reopening of the Welsh Highland to Caernarfon, a very short line.
I take it you are aware of Llandaff Cathedral at Cardiff, and Brecon Cathedral. Unaccountably I have never been to either (Llandaff especially is a major omission). Very easy to get there by train, but use Danescourt station, not Llandaff (or that is my plan anyway in my Phase 4 of the Cathedral pilgrimage).
There is also the narrow gauge steam train at Welshpool- the Welshpool and Llanfair railway.
I've been mulling this one over. I would suggest Chepstow for 3 nights then "Hereford" for 2 or 3 nights.
My rationale is that from Bristol Airport you take the bus to Bristol Bus Station then the hourly T7 bus to Chepstow.
From there it is an easy train trip to Cardiff.
But Chepstow is where you catch buses to Tintern and Monmouth.
You have a useful short cut to Hereford as the 69 runs from Monmouth to Hereford.
Buses run from Hereford to both Hay on Wye and Brecon. My preference would be Hereford. But I have a gut feeling you will be pushed towards Ludlow.

Hereford also has a free city circular bus service from the Railway Station, every 15 minutes - the City Zipper.

I like the towns and sites you intend to visit, but not sure how expeditiously you can get to and between them them using public transportation. We had a rental car.

The very short answer to the above question is to look at the Newport bus website, they run basically all the buses around Chepstow. You would be surprised how good the services are.
There is a weekly all zones Newport bus ticket for £9 a day or £27 a week. Historically it has not included the T7 from Bristol but now does. So all Newport buses from Bristol to Chepstow, Hereford, Newport and Cardiff (on the half hourly #30 shared with Cardiff bus- and of course the train is a lot faster on that axis.
Interestingly there are also National Express buses from London to Swansea via Chepstow and Cardiff.

Thanks for the replies thus far. I hadn’t considered Chepstow as I had read a few posts from a couple years back that there was some hesitation due to hotel reservations being cancelled bc of horse events and that the town was more functional than aesthetically pleasing. Obviously, one must consider everyone has their own opinion on what is pretty, but if there was an alternative that ticked that box and was pretty functional, I think we would prefer that. I will check into the Newport bus routes and the option coming into the area from London. Thanks again!

It looks to me that you will need to fly into Heathrow. That being so, assuming you will first head to Cardiff following a landing in the morning, I suggest you could consider purchasing a Super Off Peak Return Ticket as that will allow you to travel out and back within a month for the lowest price yet still allow you to use any off peak train. Of course, this means that you also return to Heathrow on an off-peak train from Cardiff - which you may not wish to do. (They are all usually off peak on weekends).https://beta.trainsplit.com will show you which trains you can get for the off-peak return fare if you select ‘Open Return’.

It would be best if you came in late June > early July. About the third week of July the schools close and hotel prices will increase and places that attract tourists will be more busy.

Cardiff can be used as a base for the local attractions such as St.Fagans Castle & Museum, Cardiff Castle, Caerphilly Castle and Castell Coch as well as the waterfront (Cardiff Bay). You can also go quickly by train to Chepstow to see the castle. However, I suggest you go to Tintern Abbey first and Chepstow Castle second if doing that as a day trip. Oh - and I would not take the footpath over the iron bridge at Tintern & up through the woods to the Devil’s Pulpit unless you are really fit. I am fit and it was hard going! Bus Chepstow to Tintern.

If you have not been to Bath, that is an easy day trip by train from Cardiff taking just over 1 hour.

Abergavenny, Hereford, Ludlow, Shrewsbury, Chester > north Wales are all linked to Cardiff by direct trains. The stations at Abergavenny and Hereford are about a 15 minute walk from the centre. Chester has a shuttle bus from the station to the city centre.
Shrewsbury would be the best location for a trip top Powis Castle - unless you are staying in Welshpool. It is a bit of a walk from Welshpool station to the castle.

In Ludlow & Shrewsbury you will find a number of ‘Ye olde England' black and white buildings.

Shrewsbury & Welshpool are on the Cambrian Line with goes through to the west coast of Wales - beware that the train splits at Machynlleth. Consider heading for Porthmadog which has 2 narrow gauge steam railways into the Snowdonia National Park. (Premier Inn is right by the steam railway station at the southern end of town - normal train station at northern end of town). Porthmadog is handy for visiting Portmeirion Italian style village.

British TV programme about the Cambrian Line.

Be sure to have your last night near to Heathrow/London/Windsor.

James has given you an out of date bus timetable for Chepstow to Tintern. Phil Anslow lost the contract to Newport Bus last year, and it is now on an enhanced hourly frequency.

I’ve been doing some reading and research this am and also talked with my daughter last night. She reminded me about East Anglia as being another area we had heard of (actually from a young man from there who was sitting beside me on the flight home from London last October) and wanted to see soon. He absolutely raved about that area and was really trying to persuade us to going there next.

Maybe I need to tap the brakes on this thread a bit until I have more sorted out with that Norfolk/East Anglia area being a possibility. It would be an easier region to get to from London, am I correct? What are your thoughts on 9 days there vs. the Wye Valley/ Herefordshire/ N Wales option?

My problem is this country is just too beautiful! I need to be able to go for three months, not 9-10 days. :)

I was in this area in April and wrote a trip report:
https://community.ricksteves.com/travel-forum/trip-reports/walking-and-eating-through-england-and-wales
It might give you some ideas. Of all the places we stayed, I thought Ludlow was the nicest town. I thought Church Stretton was the most scenic and had the best hikes, but they did involve some fairly steep climbs. Totally worth it. The transportation was fairly easy with the trains and buses, which run more frequently than I would have expected.

North Wales is lovely as well. We were there 12 years ago and stayed in Conway, Betws-y-Coed, and Criccieth. It’s not that large an area, so you could pick one base. We rented a car on that trip, so I can’t comment on public transportation in that area.

I think East Anglia sounds fantastic, but unfortunately, I have not been there so I can’t do a comparison.

Just as well that Phil Anslow has lost the Wye Valley route.

East Anglia has some gorgeous villages - especially Lavenham. The coast has some creeks - I really liked Woodbridge. However, the coast is rather boring - nothing like the coasts of the south-west and Wales. The landscape is also flat in many areas - especially on The Fens.

Norwich is the main city and would make a good hub. I did not think much of Great Yarmouth.

East Anglia has plenty of historic houses.

East Anglia is a great option. Presumably you’ll do a separate post, but a few ideas to get you started:

North Norfolk coast - seals, bird-watching, some great beaches (including the one at the end of Shakespeare in love),
Norwich - really attractive and historic city
Norfolk Broads - one of England’s best boating centres
Suffolk - Sutton Hoo, beautiful villages and towns like Lavenham, Long Melford, Saffron Walden
Dedham Vale - John Constable country
Essex - much maligned, but parts of it are very similar to Suffolk in architecture and feel
Marvellous National Trust & English Heritage properties throughout the region: Blickling Hall, the wonderful Audley End.
Oh, Cambridge and Ely too!

Yes, easy from London.

I absolutely love East Anglia- the North Norfolk coast is one of my favourite parts of the UK. You have a lot of great suggestions above to get you started. To answer your question whether it is easier to get here from London- on public transport it depends (London to Norwich is very easy, to most of the coast a bit less easy) and driving is easy once you get past the M25. Along the coast of North Norfolk however they have a good bus system. Good luck deciding.