Just because it's NT's policy doesn't mean it'd stand up in court. You'd think the reasonable thing would be to ask for permission and permission would be granted by most landowners. And then you could acknowledge them politely.
I can see that they might forbid you to take photos, that's one thing (albeit fantastically petty). But your photo or drawing or painting or poem or whatever, that's your creative work and you've got copyright on it. As I understand it anyway. They couldn't forbid you to publish and sell a drawing you'd made of stonehenge, surely - that'd be laughable. Surely that wouldn't stand up in court. And a photo is very similar if it's a work of creativity, which it is.
That;s what I reckon anyway.