Well, a small amount of Chablis or 4 cans of beer/cider are allowed.. to broaden out what you can and cannot do, EH has to my mind put up a very reasonable set of conditions....
http://www.english-heritage.org.uk/daysout/properties/stonehenge/summer-solstice/bring/
http://www.english-heritage.org.uk/daysout/properties/stonehenge/summer-solstice/conditions-entry/
You can even pick up your litter and dump it into one of several bins!! I can't see that somehow but live in hope....