Q: Are there tours to just Stonehenge or do you have to include Bath and Salisbury?
A: This company seems to go direct to Stonehenge.
Their web site says:
We operate daily tours, departing from London which include Stonehenge and explore the surrounding area – rich in History, Myths, Legends and Mysteries. We also offer a unique opportunity to go beyond the Stonehenge fences and touch the stones. Which ever tour you choose we promise a truly memorable experience.
Probably best to email them to get prices and times.