travel to Windsor and Stonehenge before tour of Great Britain begins

What time of day does your tour start? On your second day? Timing would be a little tight. I cant see how you could accomplish both on your arrival day without a rental car. You will undoubtedly want to spend the night at Bath. Windsor you can see by train from London (half an hour journey), or taxi from Heathrow. And then train (preferable) or coach to Bath. If you give Windsor appropriate time, you probably wont reach Bath until late afternoon Stonehenge you could see on the second morning from Bath- either an arranged tour or renting a car However as you arrive at 9.30 you wont be through Customs , immigration until 10.30ish- just not enough time to do them both, unless you rent a car after Windsor, drive via Stonehenge to Bath and drop your car there. You may neeed to do one of these places after your tour.

Sorry, yes, tour begins on 2nd day at 5 pm. Was concerned about dragging luggage to Windsor from LHR and then how to travel from there to Bath, or whether we should simply stay somewhere overnight in Windsor or London and travel by Stonehenge the following day onto Bath? Maybe I'm just making this more complicated than it needs to be and I appreciate your suggestions.

OK. Absent renting a car: Train from Heathrow to paddington. Leave luggage in a "Left Luggage" facility at Paddington. Train to Windsor. Lunch in Windsor. same train bck to Paddington. (no reservations needed). Pick up luggage- first available train to Bath (shouldnt need reservations). Advance booking may save some money on train fares, but then you may be locked in to specific trains If you overnight in Bath you can take one of the tours from Bath to Stonehenge (and possibly a few other sights). (Mad Max is usually recommended here). back to Bath in tiime for start of tour. (I would overnight at the hotel where the tour starts, so you dont have to worry about moving luggage). Plan 2 If you rent a car it would be a whole lot easier. rental car from heathrow to Windsor (parking is a little tight, but if its mid week it should be OK, couple of parking lots in town) Drive to Stonehenge is relatively easy once you are on the motorway going West. Plan 3
You could also mix the two above, Windsor and train to Bath. On second day rental car to Stonehenge, Avebury etc etc etc

We spent a night in Windsor on our last trip and I highly recommend. We took the bus from LHR as we were advised the taxis charge very high fares. The bus dropped across the street from our BandB and we spent the afternoon and evening strolling alongbthe Thames path and walking through Eton. Had a wonderful dinner too. In the past I have taken the train from Bathnto Salisburynto visit Stonehenge. That was very nice too---we rented bikes to ride out to Stonehenge along a country road. I think you could easily do one or the other, but doing both might get complicated, and expensive.