EH make you sign a waver or pay a fee when taking photos inside Stonehenge.
There was a long discussion on BrtArch a while back about how EH would go about trying to charge people who break the waver.
The Negotiating Avebury Project made me the official Chronicler for the Avebury digs over the last 6 years.
For commercial filming inside Avebury for film or TV broadcast you must have the pemission of the NT and the local council. Needless to say there is a fee.
May people have filmed inside Avebury without permission but the trust has never taken anyone to court about it. There are no signs saying No Photography so I doubt any case would stand a chance in court.
As you say, on private land always ask permission. Just good manners innit?