The things you learn watching QI actually are quite interesting. Here in the UK VAT is not charged on cakes, or on biscuits unless “wholly or partly covered in chocolate (or some product similar in taste and appearance)”. This led to Her Majesty’s Revenue and Customs (née Customs and Excise) investigating the Jaffa Cake. The ruling rested on the fact the biscuits go soft when stale, whereas cakes go hard when stale. Jaffa Cakes go hard and are therefore cakes, and not subject to VAT. The clue’s in the name.
For those who want the full rules, you might want to look at HM Revenue and Customs Notice 701/14 (PDF) which helpfully clarifies that VAT does apply to “any item of sweetened prepared food, other than cakes and non-
chocolate biscuits, which is normally eaten with the fingers.”