Think the House of Commons ruled they were Cakes (just making this up). There is some better tax deal if they are cakes (again - source = made up).
Here are the conclusions of that debate:
https://www.gov.uk/hmrc-internal-manuals/vat-food/vfood6260
There were rumblings a year prior, and it was also cited in a 1994 trademark dispute.
We had a branch of The Jaffa Cake Appreciation Society at university.