Down here, a large piece of bread that you have sliced (or slice yourself) is a loaf. Fastfood outlets put burgers in buns, but locally a bun is a cake (like a current bun). A small, round piece of bread is either a bap or a roll. A roll can also refer to the entire product including it's filling (i.e. a round version of a sandwich). A small, long piece of bread is also a roll (commonly holds a sausage).
A cob is a small horse. A batch is a large number of anything, not limited to bread, but I believe so-called because "I've just made a large batch of bread".
A Barm is not a word in the south west AFAIK.