It is interesting to note that in the UK the term "bap" is often used with burgers and sandwiches served on a soft roll like a hamburger bun in the USA, whereas a sandwich on a hard roll is a called a baguette. So in the morning you might go to the train station and there is a guy there selling an egg-bacon bap for travelers. UK English speaking countries seem to know this term but in my travels I have seen it most often used in the south of England. I don't have any specific knowledge but it seem likely that "bap" is like lift/elevator or lawyer/solicitor just a word choice that was popularized by common usage, and based on the observation on the frequency of usage most likely from the south in the UK.