I can think of very few traditional singers who used dynamic; those who did usually had a reason for doing so.
"Davie Stewart, Willie Scott and Phil Tanner."
Davie Stewart's singing was, as with others in his situation, influenced by the fact that he was a street performer, used to having to project his voice in the open air and often over traffic noises, in a constantly altering environment.
Margaret Barry was probebly the archetype of this style of singing.
Phil Tanner and Willie Scott - certainly not in a million years.
Sam Larner used it because he 'performed' his songs, playing his audience pretty much as a music hall performer did - not entirely absent from the tradition, but not common.
IMO, at best, a singer will use subtle tonal rather than volume changes, singing with an evenly controlled delivery.
The revival style that SO'P describes with singers like Bellamy owe more to a theatrical approach and personal idiosyncracies rather than the tradition.
As far as English traditional singing goes, I tend to think of dynamic in terms of volume and alteration of pace; but then again, most of our examples are of singers remembering songs rather than performing them within a living tradition.
Jim Carroll