" it doesn't seem like the Uffington mistake was part of any sort of 'policy' "

Well, it does seem to be part of a pattern, at least.
If they consider it was a "mistake" it wouldn't be hard to put some policy guidelines together to avoid repetitions. But if they don't do that, and it happens again, it's a policy in all but name.

The new management was Fiona Reynolds, new Director General in 2001 I think. I'm sure I read a "new broom" speech by her moaning about finances and saying one thing would be to generate income in innovative ways out of their properties. Then this stuff started. Letting out their castles and stately homes for filming has become a real earner for them, and they have a promotional web page about it. Perhaps no-one has thought to make a distinction between stately homes and white horses etc when it comes to filming. I'd have said there's a hell of a difference as in the case of the latter there's an element of violation of people's feelings of respect.

It's also, IMHO incredibly unwise to do it to a monument that's up on a hill and so visible, a. because it's such a crass affront to the aesthetics of the place and b. because the NT of all people is saying look, we have zero respect for this thing.

Most peculiar of all is how come they have people in their ranks that aren't sensitive to those points?