One other note: Stonehenge is worth seeing, but it's a good 1.5hr bus ride from London (about the same for train + shuttle). They built a new visitor center which is much nicer than the old one, but it's nearly two miles from the actual Stonehenge. You either walk the two miles via footpath (which my older son and I did), or you wait in a queue for a shuttle bus to take you to the stones (my wife and younger son chose this option).
We did Stonehenge as part of a Viator day tour to Windsor Castle, Stonehenge and Oxford. We wished we had more time in each spot, but it was a good option since we only had a week there. My caution: Our tour got us to Windsor before opening time so we got a spot at the front of the queue to get in. By the time we left 2h later, the line was several hundred people long, and I doubt those folks on tours got much time at all to spend in the castle. And in the summer it'll be worse - we were there in early March before the full tourist wave really hit.