Older compo lovers may remember the joy of food items cunningly disguised as tubes of toothpaste. These were:
Margarine: essential component of the mess tin devastating Bacon Grill fry up ('oh, that was your mess tin was it? Soz!'), but sadly unable to unite biscuits AB and Cheese Possessed in any sort of edible combination.
Condensed Milk: a sticky liquid so sweet that it may have been the reason that diabetics were prevented from serving their country. No need for sugar in your brew, this would cover it. Need a serious sugar rush and no time to cook and unable to face un-sticking a boiled sweet from its wrapper? This would do it. A multi-purpose substance, which doubled as Nectar of the Gods, so it had to go, replaced by powdered milk (ack!) and dextrose tablets.