The contracts between Amazon and publishers set the minimum price that the retail customer must pay.
Think of it as price fixing, because it's a legal version of that.
Does not only apply to books. A company selling high end fashion, or hifi, will contractually force retailers not to discount below a set floor
So Amazon has agreed contractually to sell book X for ŁY.
Changes to vat do not affect that contract. If vat had gone up, to say 30%, Amazon could / should still have sold for ŁY, that's the deal they made.
I don't like it any more than you do. If you want to organise a no win no fee class action, sign me up
