With regards to copyright protection of photos in galleries under Australian law, I've just been reading the Copyright Act. Under the act, an "artistic work" is defined as:
(a) a painting, sculpture, drawing, engraving or photograph, whether the work is of artistic quality or not;
and several other things.
Section 31 of the act then goes on to state that:
For the purposes of this Act, unless the contrary intention appears, copyright, in relation to a work, is the exclusive right to...
<snip>
(b) in the case of an artistic work, to do all or any of the following acts: (i) to reproduce the work in a material form; (ii) to publish the work; (iii) to communicate the work to the public;
If you took the above literally, that would appear to award copyright protection to photographs of artworks.
However, one little bit of law I picked up recently is that whatever the literal meaning of specific words in legislation, there is something called the doctrine of purposive construction, where judges try and figure out what the law was *intended* to say rather than what it actually *says*.
You might be able to mount an argument that the clear intention of that reproductions was to protect works where some creative effort was made, rather than merely protect an exact facsimilie of a creative work now in the public domain.
But then again, maybe you mightn't: IANAL... :)
I suspect the law in the UK is similar to the Australian situation and this is what the British National Portrait Gallery is basing its right to be narky on.
Of course, just because a work might be protected by copyright in Australia or the UK doesn't mean it's copyrighted in the US - so it it's quite possible it might be legal for an American Wikipedian to download and add those Shakespeare images to the gallery, as long as they're not going to visit the UK any time soon ;-)