I don't know if this document has been mentioned on here before-

"UK Photographers Rights Guide"
which can be downloaded as a pdf from
http://www.sirimo.co.uk/2009/05/14/uk-photographers-rights-v2/

There's some interesting stuff in there, particularly on the issue of trespass, but in relation to Stonehenge etc here's a couple of quotes-

"It is not an infringement of copyright to take photographs of buildings, sculptures and works of artistic craftsmanship that are permanently situated in a public place or premises that are open to the public"

but on the flip side it also says about private property-

"the owner has the right to impose whatever conditions he wishes on entry to his property, including a restriction on photography. Photography is prohibited by the conditions of entry to many museums and stately homes for example"

It would seem to depend on whether Stonehenge was regarded as a 'public place or premises that are open to the public' or whether it is classed as private property belonging to EH who can then impose restrictions on commercial photography. I would assume that there would have to be notices up at the entrance telling visitors that commercial photography was not allowed - I don't remember ever seeing any. An interesting upshot of this is could be that once you'd paid yer money to go in then you couldn't sell any pictures you take, but if you stand by the side of the road taking pictures then you can do whatever you like with them.
I have in my hand a can of worms labled 'who owns stonehenge' - anybody got a tin opener?

-Chris