Something caught my eye on the ISS web site, so I looked up "deli" in Wikipedia, the fount of all knowledge:
Delicatessen is a term meaning "delicacies" or "fine foods". In English, "delicatessen" originally meant only this specially prepared food. In time, the delicatessen store where this food was sold came to be called a delicatessen, and in this sense is often abbreviated to deli.
Then I looked again at the Waddington ISS site:
One Deli Meal Deals at lunch include the 3's Meal Deal at £3.10; great value for a baguette, a packet of Walkers crisps and a bottle of Pepsi.
Ah, those delicacies, Walkers crisps and Pepsi!