Sad but true I know the answer to this one.
VAT is payable on biscuits but not cakes. The Vatman argued they were biscuits as that was where they are stored in the supermarket. McVities won though saying they were cakes - the scientific experiment in court was that cakes go hard when left out of a tin and biscuits go soft. M&S used same argument a couple of years ago re their chocolate teacakes (Yum).