As above. But it really depends on what kind of crowd you'll have as well, and what kinds of activities/attractions they'd be interested in.
A word of warning about Stonehenge though. As historic as it is, it's not what you'd expect. In order to protect the site, the authorities won't let you within about 30 meters of the place, and you certainly won't be allowed to get close and touch the stones. you can go there, but it'll be interesting for all of 5 minutes. For stone circles, I'd recommend Avebury, which is bigger (it has a whole village inside the main circle), and there's no restrictions about you getting close and touching the stones (and the various surrounding monumental arrangements as well).
It really depends on where you'll be based, England's not as small as you might imagine and it takes time to get places.