Have not been on the RS London tour but have done Istanbul and Florence RS tours. Hotels were fine. Clean, safe, great locations. Do not recall strenuous walking on either tour.
I prefer to travel solo.
Will speculate the RS tour hotel will be fine. Just know stairs are a staple in the UK. I’ve always contended its the reason for the longevity of populous.
As far as walking, yes, there will be some. IMHO walking is best way to appreciate London so definitely bring comfortable worn in shoes.
Believe you can look at each days agenda and it will state how much walking will take place.
Stonehenge can be seen on your own. Train from Waterloo Station to Salisbury, board a bus to transport you to the Stonehenge Visitor Center. Pay admission and follow the path. Bus back to Salisbury station, train back to Waterloo.
Definitely arrive a few days before or stay a few days after the tour to explore on your own. Shop, eat, visit free museums, walk along the Thames Path, enjoy sitting on a bench in a park people watching, meander about a cemetery, see theatre. Throughly enjoyed seeing Witness for the Prosecution at County Hall. Staged in a real courtroom.
https://witnesscountyhall.com/
Great city. Enjoy it!