Taking our traditional singers as a guide, the folk voice is the one you speak in, as is the folk style of narrative - i.e. singing in speech patterns (taking the breath on the punctuation).
Without wishing to be critical of the lady, Shirley Collins does not use a natural voice for singing - she constantly sings in head-voice.
Head voice, which appears to be confined to women singers, creates a number of problems:
a. It takes twice as much air to produce, making it difficult to sing a full line without taking a breath in the middle.
b. Invariably, if a song has a largish range a singer can often have difficulty in maintaining a single tone and will move from head voice into chest voice as the pitch moves up or down.
Jim Carroll