I used http://www.premiumtours.co.uk for the exact trip that you are considering, Stonehenge to Bath. Make sure you book the early morning trip to Stonehenge that allows you to actually go inside the circle, as actually getting up close to Stonehenge is prohibited for most tourists, unless you are with a tour company that has a scam going with the historical society.

It really is a meaningless trip to Stonehenge if you can't be up next to it.

The company overall was very good. The guide was very knowledgeable and friendly. My only complaint was that we were rushed in Bath, which really is a beautiful city with some neat shops.