UK 14 days

If you've already book everything my comments might not make much difference. Touring Windsor the first day might be a good activity. You could take a short cab ride there. You could take the train back to London. On day 5 you could take the train to Oxford and pick the car up there. It's almost 2 hours from Chipping Campden to Bath. That's kind of a tough day trip IMHO. The RS recommended trip starts in Bath. The drive from Chipping Campden to Conwy is a little over 3 hours. Conwy Castle, Caernarfon Castle and the Snowdon Railway are the big sites. York to Edinburgh is also about 3 hours of drive time. Hadrian's Wall is about an hour detour of the most direct route. Bamburgh Castle and Holy Island are sights on the coast. Hopefully, you're getting rid of your car right as you arrive in Edinburgh. Take the train to Glascow.

I will follow this string with interest since Mary and I are in the UK (third time) for 4 weeks beginning August 12. Mary will be doing research collaboration in Southampton and Bristol during first 2 weeks, then a conference in Glasgow and then we drive around the Highlands. I see that Jersey Boys is still playing in London. I'm from New Jersey and I got a kick out of the English actors trying to talk like we do in New Jersey. Not that I talk like that. If you like the Four Seasons, then you would probably enjoy this show. Tickets for shows in London are often quite a bit less costly than Broadway shows. You can even make deals at some of the theaters. There is a pub next to the theater where Jersey Boys is playing right in the heart of the theater district near Leicester Square (with a discount tickets booth) which is just a block or 2 away from Trafalgar Square or Piccadilly Circus. I remember that our first major stop in London in 2003 was The British Museum and we were quite impressed as so many visitors are. The music at St Martin-in-the Fields Church on Trafalgar Square is amazing. Excellent progressive theology and compassionate ministry too! If you like to meet fellow travelers and enjoy a good tour, I recommend the Mad Max tours from Bath as does Rick. They do Avebury, Stonehenge, Cotswolds. Enjoy!

Sally: With a 7:30 am arrival at LHR, you will probably make it to Paddington....you referred to "train" , so I assume you are taking the Connect or the Express, which go to Paddington....probably at around 900 am, give or take. So you could probably drop your bags at your hotel and be ready to do things before lunchtime. I strongly recommend lots of walking and outdoor events on your first day. Exercise and sunlight...the latter always a question mark in London... really help jet lag. London is a great city for walks. Consider doing Rick's Westminster walk first thing to really feel that you are indeed in London. From Trafalgar Square, walk over to Covent Garden and eat lunch, ideally at an outdoor cafe/restaurant. In the afternoon, spend some time strolling through Hyde Park and Kensington Gardens, and perhaps do the London Eye/South Bank toward dinnertime. Turn in early, and you should be more or less energized the next morning. I second Hadrian's Wall as a stop going to York. Stratford on Avon is extremely touristy, but everybody should do it once. If you have an opportunity, you might train (not drive) into Coventry to see the ruined medieval and post World War II new cathedral, as it is quite a moving experience.
Since you love history, etc, be sure to hit the Tower of London early on your second day in London. Get tickets using RS advice and be there when it opens. Plan on a good half day. The other London essentials I will leave for other posters. Have a wonderful trip!

Day 6. After Bath, South Wales is really out of the way. Probably not realistic. Spend time back in the Cotswolds instead. You may want to try for tintern Abbey (ruins) in the wye Valley. A mystical spot and not too far a drive back to the Cotswolds. Sat 10.Conwy to York- Chester is an interesting town for a stop (including walls and Roman ruins). Otherwise maybe touch on Derby or Lake district Iron Gorge- Blists hill is an interesting recreated victorian (?) village with preservef shops etc

I like your itinerary. I would do some sort of walking or biking tour your first day in London to keep you moving and give you a nice overview of the city. And I would consider making that first night out of London an overnighter in Bath (maybe add Glastonbury/Wells to the mix). Oxford would be a reasonable stop between Windsor and Cotswolds, although you could stay for Evensong in St. George chapel and just spend the whole day there.