What food do people normally pack?
I usually just take some cereal bars or similar (Kind bars etc), not usually organised enough to buy or make sandwiches, so just get a sandwich at the airport (before or after security) and something to drink once through security. Expensive but still a lot cheaper than food on the plane, Edinburgh airport has a WH Smith past security so you can get a meal deal (sandwich, drink, snack for about 4 or 5 pounds I think it is). Most UK airports will have a Boots or WH Smith or similar past security and a supermarket like M&S or Tesco before you go through as well as the usual Costa, Eat, Pret or similar.
No travel pillow or clothing change for short-haul (Ryanair you can take both your bags on board anyway with priority boarding so no risk of lost luggage).
My small ( under seat) bag has tablet, headphones, camera, travel toiletries, snacks/food and drink plus battery pack and cable for phone/tablet, passport, etc, sometimes a paperback book too.