I think the point being made is: the photographs were created with the intention of being as faithful to the original paintings as possible without adding any new creative input. Doing that in photography is hard work, an incredible amount of time and expertise goes into the setup for the picture.
In the US, such a faithful duplication of a work that is in the public domain is also considered part of the public domain. The point isn't that the duplication was necessarily easy to perform, but that the duplication was performed with the minimum possible amount of new creative input so as to remain as faithful to the original as the duplicator could manage. This doesn't mean that any image that incorporates source material from the public domain is automatically impossible to copyright, but it does mean that in order to copyright it the new work would have to be something other than a maximally-faithful reproduction of the original.
You're right, the creation of the copies likely involved a significant expenditure of time and effort and requires a great deal of skill. However, in the US, the amount of effort required to do something has never been part of the test for whether a work can be copyrighted.
The fact that the portraits are in the UK, and the work was done by a UK photographer at the behest of a UK organization means that, in the UK, those works may be considered copyrighted. However the contention thus far has been: in the US they're not considered copyrighted.