Bit more info from Wiki....
Under UK law, no VAT is charged on biscuits and cakes they are "zero rated". Chocolate covered biscuits, however, are classed as luxury items and are subject to VAT at 17.5%. McVitie's classed its Jaffa Cakes as cakes, but in 1991, this was challenged by Her Majesty's Customs and Excise in court. This may have been because Jaffa Cakes are about the same size and shape as some types of biscuit. The question which had to be answered was what criteria should be used to class something as a cake or biscuit. McVitie's defended the classification of Jaffa Cakes as a cake by producing a giant Jaffa Cake to illustrate that their Jaffa Cakes were simply mini cakes.
They also argued that the distinction between cakes and biscuits is simply that cakes go hard when stale, whereas biscuits go soft. It was demonstrated that Jaffa Cakes become hard when stale and McVitie's won the case.