Which is best?

It's pretty easy to visit both Stonehenge and Blenheim Palace as day trips from London. For Stratford-upon-Avon I'd recommend staying overnight both as the journey from London is a bit long (no direct trains) and because there's a lot to see. For Stonehenge if you want to also see Salisbury you might also want to stay overnight, tho you'd have enough time on a day trip to see both Stonehenge and Salisbury Cathedral.

IF you are coming early, I'd go to Salisbury for at least one night (maybe 2 if you have the time) before you head out to Bath to pick up the tour. Do your Inner Circle AM from Salisbury then you will just have about a 15 minute drive to the stones and not a couple of hours. I would also want to arrive in Bath the night before the tour start. There is a lot to see there that is not covered on the tour.

I had to go look at the itinerary for this year as on my long-ago tour we went to Blenheim Palace. I see they have changed it to Oxford which is probably a good choice. I liked Blenheim Palace but did not love it...BUT it was also raining cats and dogs so couldn't really see the gardens. Unless you have an interest in Churchill and want to see where he was born, you might consider a day trip to Hampton Court Palace from London which is older and I found more interesting.

Your trip, your choices, though so do what makes you happy!

Thank you Pam and Cat. Much appreciated.

We did Stratford Upon Avon and Blenheim Palace as well as Oxford on day trips from Chipping Campden in the Cotswolds.
Be sure to go to Shakespeare's birthplace in Stratford and take the Shakespeare tour that includes the site of his home (home no longer exists), the church with is tomb an his wife's home. You need at least 4-6 hours there.

Not sure about doing Stonehenge at dawn.

You could go straight to Oxford, then stay for 2 nights-
First full Day in Oxford- Stratford on Avon (train to Leamington Spa, change for train to Stratford on Avon).
Day 2- Blenheim Palace by bus, you could transfer to Salisbury that evening-
hourly train Oxford to Basingstoke, change, half hourly train to Salisbury
Day 3- Stonehenge and Salisbury ,
half hourly train to London.

There is a daily National Express bus at breakfast time direct from Salisbury to Heathrow (continuing to London Victoria).

From Oxford it is possible to go by local bus to Salisbury- the S6 bus every 20 minutes to Swindon, then the hourly X5 bus Swindon to Salisbury- takes longer but you see more of the countryside.

The other route by train (apart from frequent service via Bath) is Oxford to Didcot Parkway, change, train to Swindon, direct train Swindon to Salisbury- one early evening service.
On a Saturday there are also two direct trains from Oxford to Bath, change for Salisbury.

Thanks a million Geovagriffith and isn31c! You've all been a great help.