Cake: A sweet, baked confection usually containing flour, sugar, flavouring ingredient, eggs and baking powder or baking soda.
Biscuit: A sweet, normaly hard baked 'bread' of flour, butter milk, baking powder and a shortening agent.
It seems from looking at several recipes, biscuits may or may not contain eggs, whereas cake is rarely is devoid of them.
Can someone check the box of jaffa cakes to illuminate this point as that may clinch it.
Incidentally biscuit is from the French bis cuit 'cooked twice'.