"Duration of copyright
The 1988 Copyright, Designs and Patents Act states the duration as;

For literary, dramatic, musical or artistic works
70 years from the end of the calendar year in which the last remaining author of the work dies.

If the author is unknown, copyright will last for 70 years from end of the calendar year in which the work was created, although if it is made available to the public during that time, (by publication, authorised performance, broadcast, exhibition, etc.), then the duration will be 70 years from the end of the year that the work was first made available."

So in the case of prehistoric rock art, the copyright owner is unknown, so any copyright would have expired about 4,000 years ago. It doesn't vest with the current "owner" of the land the rock art sits on.

In the case of a photo of a prehistoric monument, not taken in the course of employment, the copyright is with the taker of the photo until 70 years after their death. Then it goes "public" - i.e. it doesn't vest with the NT.

Ipso facto, they're talking bollocks!