Never taken such a tour...BUT...
We did get held up at Heathrow...thank you, Swiss Air ...for lost luggage (which they eventually delivered to me three days later in Mold, North Wales...(and Mold is what I must have smelled like after wearing the same clothes for three days)...
Anyway, we got to Stonehenge "after closing"...You drive up, turning right off the main motorway and there's a parking lot on your right...there's a guard there, putting up a sign which reads "Stonehenge closed"...
Amazing since it sits out there on Salisbury Plain, out in the middle of nowhere...
But, it is surrounded by a "cyclone" wire fence...
So, we parked the car off on a side street and walked back along the fence, taking pictures from, over and through the fence...You can actually get close enough to get decent pictures, you just have to shoot around the impediment...
Well, during this adventure, we never saw any tour groups being let into the enclosure...So, I wouldn't know what this tour entails...
Funny thing is...I've seen acouple of other folks pictures of Stonehenge since...and both of these folks obviously took their shots all through the fence as well!!!