Jaffa Cakes are actually cakes. McVities (I think it was) created a 12" (or something) Jaffa Cake to present to a board to sway their decision of whether Jaffa Cakes should be considered cakes or biscuits. They wanted Jaffa Cakes to be classified as cakes because it puts it in a lower tax bracket than if they were considered biscuits. If it was considered a biscuit it would be put in a higher tax bracket because it has chocolate on, and biscuits containing chocolate are classified as 'luxury items'. The difference between a biscuit and a cake is that biscuits go from hard to soft when stale, and cakes go from soft to hard when stale. The 12" Jaffa Cake served to show that said Jaffa Cake went hard when stale :D and was therefore classified as a cake, and not a biscuit.
Unfortunately, the British government want to reclassify it as a biscuit...meaning we'll have to pay more for our beloved Jaffa Cakes!
*N.B. info may not be all correct as it's done from memory.