Be careful about a general assertion of copyright. As only the creator of the image has a copyright (unless assigned under contract), and since the pictures were apparently taken by an enemy of the U.K. , it would be hard to say that a copyright exists.
If a Crown subject took the picture, that would be a different matter.
Many collections assert a general copyright, then you have to establish the exception. Makes it easier for the museum.
Finally
, without the specific facts surrounding that image, it is all speculation anyway.