The Guardian on the UK version of the deaccessioning debate.
It includes the following: "In England, the rules about these matters are laid down by the Museums Association’s code of ethics, used to set Arts Council England’s standards, and they state that artefacts 'should not normally be regarded as financially negotiable assets'."
Obviously that puts a lot of weight on normally. In the U.S., by contrast, the "ethical" rule is artworks should not normally ever be regarded as financially negotiable assets unless the plan is to use the sale proceeds to buy other artworks in which case feel free to regard them as financially negotiable assets.