According to this piece by Tiffany Jenkins, the rules for deaccessioning in the UK are different than they are here:
"Two years ago, the Museums’ Association, the professional body for the sector, significantly altered its policy, softening its attitude towards the protection of the collection. ... [T]he MA came out and argued for the first time that financially motivated disposal was permissible in certain circumstances - not just in order to buy another work, but also to raise cash."
Who knew that British museums were repulsive Stalinists? Why oh why won't the AAMD do something about this? Shame on the Royal Cornwall Museum, shame on the Museums' Association, shame on everyone!