English folk song is in those respects no different from Scottish and Irish folk songs, and their is much crossover between all.Carthys retuning of the guitar was inspired by Davy Graham who was influenced by the Oud , I understand he came acorss that instrument in Morocco.
    other guitar tunings are influenced by the 5 string banjo eg dgdgcd used by martin simpson. dgdgbd which is a five string banjo tuning.
I am very wary of people using the term English as if it was something that has no outside influences., because of the possible nationalistic connotations
English music is no more odd, thah Appalachian Scottish or Irish, THEY all use the same tune modes Mixolydian Dorian and Ionian and very occasionally aeolian
The leader of the National Front Mr Griffin purported to be intersted in english folk music,presumably as he was an english nationalist