Not to be confused with a sandwich, a butty is formed by folding a single slice of buttered bread around whatever takes your fancy and is readily available (i.e. requiring no further preparation). In its purest form a true butty is filled with ingredients, in any combination, from a meal already underway. For example, if you are eating a "Full English" breakfast, you may butter a slice of bread and pile on any one or more of the items on your plate (e.g. egg, sausage, beans, black pudding, bacon, mushrooms). No food is excluded as a potential ingredient of a butty provided it is ready to eat at the time. The all-time favourite though must surely be chips (as known in the UK, french fries elsewhere).
Gorging on a butty is always a more joyful activity than eating a sandwich as it embodies the pleasure of mild gluttony, rather than being a technical exercise in meeting nutritional needs. The contents are selected in a spontaneous fashion and usually involve a certain excess, preferably challenging the structural integrity of the bread and hand control / oral capacity of the consumer.
Parent: "What did you have for lunch?"
Child: "Just a jam butty."