Songs were spread by travellers, fishermen, miners etc . all over. because a certain collector happened upon it for the first time in Cornwall, or anywhere else, gives us no clue as to its origin. presumably Cornish victorians understood 'coals to Newcastle,' and could include it in their speech, without even being aware of exactly where Newcastle was, never mind having seen or heard 'it'.
Similarly, the metaphors of folk song could be appreciated wherever they originated and wherever they were adopted.