Certainly the GNU FDL and Crown Copyright are incompatible. So Crown
Copyright provides no reason to think that we can use this is in
wikipedia. There could be other rationales (fair use in the United
States for example), but I think we should try in almost all cases to
avoid those and instead use freely licensed images for all cases other
than those of extreme historical importance where we have no other
choice but to rely on fair use.
--Jimbo