The British Library Puts Over 1M Images in the Public Domain
openculture.com
openculture.com
http://britishlibrary.typepad.co.uk/digital-scholarship/2013...
Their status as images belonging to the public is not related to their being made accessible by the British Library.
See the questionable German Projekt Gutenberg: https://en.wikipedia.org/wiki/Projekt_Gutenberg-DE
Is there a proper English language discussion of any court case in which this has been tested for this particular claim?
For reference this http://www.jcms-journal.com/articles/10.5334/jcms.1021217/ is quite a good review of the NPG vs Coetzee non-case [English law] and the Bridgeman vs Corel case [USA law].
In particular the BL position is that due to the proprietary scanning technology they use they inherit copyright on the new works. You could potentially challenge that position in court, but at the moment there's no evidence for how a court will treat it.