Charles Ellson wrote

"Prohibited place" signs on the barbed wire outside a military base
might, but I don't think the ones (eg on the doors to checkin hall)
at
civilian airports purporting to ban photography do. The signs in
(eg)
immigration purporting to ban use of cameras and mobile phones don't

either.

Non-military and indeed non-Govermental installations that could be
declared Prohibited Places under the Official Secrets Acts included
railways and seaports so it is quite likely that airports have been
added since 1920.

I've tried Googling gov.uk with various combinations of words to drag
out some kind of specific regulation to back up such signs in
airports
but I don't get anything to suggest that such signs are other than
DIY
efforts with no supporting regulation.
There are such things as :-
http://www.pressgazette.co.uk/story....&sectioncode=1
which suggest that BAA seem to think they have some God-like powers
which I would bet at least a fiver on the local magistrates telling
them they haven't.
Maybe I should take a photo of one of the notices next time I see
them -
from the public side of the line in the sand, obviously

Hee. Since even that might be illegal.

--
Mike D