if one analyses musically, most of the tunes in the southern english folk repertoire that i hear played they are in the major key, a very small percentage are in the dorian mode.
irish music uses the dorian mode, the major key, the mixolydian[flat 7] and to a lesser extent the aeolian mode
northumbrian english music, has more of a scottish influence and uses mixolydian, dorian, and the major key
Shetland music is intersting too, particularly the harmonic influence of guitarist, peerie willie, who learned his style from listening to jazz guitarist eddie lang., on shortwave radio during the second world war
a wonderful example of crossover between jazz and trad.