Like most queries connected to the living and forever changing nature of language, there is no simple answer to you and your friend’s dispute. A quick rummage through the Oxford English and we can define ‘butty’ described as a slice of bread and butter or a colloquialism for a miner paid on a fixed amount per tonnage. Likewise ‘sarnie’ comes up as ‘informal British English for a sandwich’. As to which term ‘most Lancastrians’ would utlise in their munching of said bread and chip delicacy is open to great debate - until or unless a public users chip shop survey is implemented. LHL has no wish to cause any culinary controversy but drawing a line East from Blackburn to Colne I think one would find that ‘butty’ comes out as the informal term most likely used. This is not to say that ‘sarnie’ may well be dominant in other zones of the red rose county!