My own understanding is simply that a "buttie" is a "working class" term for anything served in a bun, roll, bap (depending on which part of the country you're in) or between two slices of bread.
The more "sophisticated" members of British society, while still enjoying a buttie, would refer to a filling between two slices of bread as a sandwich and, if it were in a bun of some sort, it would described as a roll.
The class system still thrives in the U.K. and it's in the little things like food terminology that it shows up.
All this typing has made me hungry, I'm off to make meself a chip buttie with lashings of ketchup and a pint mug of tea.