"There are two types of music that I would welcome in any folk club. One is traditional folk music and the other is contemporary music in the folk idiom."
My argument from day one Dave - I've goven myself typist's cramp repeating it
The problem is your interpretation of what constitutes 'the folk idiom'
Our folk songs are basically word dominated, narrative communications of ideas and emotions - accompaniment, where it occurs (which is hardly ever in the case of British singing) is secondary to the narrative
The characters in the songs are identifiable people, usually with occupations and individuality, not the identityless ciphers of pop songs
They have problems and situations we can all identify with, which is why our folk songs have lasted as long as they have
Structurally they ar four or eight line versified, they may have choruses but they hardly ever repeat phrases other than as a plot device - unlike the endless and usually mindless over and over repetition   
That, in my opinion, is 'the folk idiom'
How the hell does your Ed Sheeran or The kinks... or much of what passes fro 'contemporary folk" fit into any of that
The original list is on another thread I think - I'll dig it out if you insist, but I can assure you, you have not responded to it despite my requesting you to do so several times
Jim