Our tour begins in Bath. Better to get to Bath a day early and then tour Stonehenge?
Yes. This has been thoroughly answered above by Pam and other forum members.
However, I'd like to add...
You can take a coach (bus) directly from Heathrow to Bath upon arrival. There's no need to go into London and back out again, unless you just want to.
I would add two days in Bath before your tour starts. This will allow you to take your (Mad Max) trip to Stonehenge (mentioned by Pam), plus have a day to walk around Bath on your own, exploring. You may wish to go inside Bath Abbey. The RS tour does a walking tour of Bath, but does not include going inside the Abbey.
There's a lot to see in Bath; the Holburne Museum, The Museum of East Asian Art, The Costume Museum, Pulteney Bridge, lovely parks, The Museum of Bath at Work, The American Museum, just to scratch the surface.
If you're taking the Rick Steves Best of England tour, it ends in London so you could build on extra days there then to explore more of London.