I went with Golden Tours without making an advance reservation, just walked up to the counter at Victoria Coach Station and bought my ticket (granted, it was Dec 30, so it wasn't high tourist season). I had a good time, I declined the included lunch option and there were plenty of restaurants and cafes in the vicinity of Windsor Castle. I like to stop and read/stare at things and take lots of photos, and I felt that I was given adequate time to do all that without fear of having to race back to the the bus and get left behind. Some companies can pick you up near your central London hotel if you are along a major route in the morning, I didn't look into that. They did have a few drop off points at the end of the tour that differed from the morning route, but were near tube stations that worked out fine for me as a solo traveler.

The tour of the Romans baths required an extra admission fee (Golden Tours only brought us to the city itself, maybe other tour companies include the ticket), which I paid at the door, and I also brought an empty soda bottle with me to fill up with "Bath" water and bring back to share with the rest of the group who didn't pay to enter. Entry into Stonehenge and Windsor Castle was included in the tour price.