Places to visit outside London

Three hours if you are lucky.

Windsor is near airport and shouldn't be missed.

Are you planning these as day trips from London, or as a sequence of one-nighters in 3 locations?

If you do Stratford-upon-Avon as a day trip from London, you should get the earliest possible start and plan on returning late -- in other words, it will be a long day. You say you'll be driving; does that mean you're renting a car for the 3 days? S-u-A encourages use of the public parking garage next to the TI where the HoHo bus is based. It's possible to drive yourself to the out-of-town sights (Anne Hathaway's cottage, Mary Arden's farm) but the town is prone to traffic jams and navigation could be a pain, not to mention limited parking at each stop. If you try to see S-u-A in less than a full day, I think you'll come away frustrated at not having seen much. We allowed a full day, including tickets to a play at the Royal Shakespeare Theatre and overnight in hotel. It still felt rushed.

I would suggest starting farthest from Heathrow (i.e., S-u-A) then working your way south. If you spent your last night in Glastonbury, you could get a feel for the town in the evening and then drive out right after breakfast. In timing your drive to the airport, allow extra time for traffic jams and the possibility that you'll take a wrong turn somewhere along the way.

I'd take Bath or Gloucester over Cirencester and Windsor over Glastonbury. It depends what you like! I'd also go to Brighton. It's tacky but Georgian and gorgeous British seaside experience!

Stratford-on-Avon / Oxford :/ Cotswolds would be my choice.

Bath / Glastonbury / Salisbury would be great too.

Thanks to all who responded