Public domain - it is or it ain't?

Alright, so I want to use this picture from WWII in a film, according to wikipedia it's public domain.

https://commons.wikimedia.org/wiki/File:Royal_Scots_with_flag_01-1945.jpg

However if you follow the link the Imperial War Museum website they say you need to purchase a licence for it.

https://www.iwm.org.uk/collections/item/object/205205406

So, what's the deal? Any ideas?