"the subject matter, the partisan nature towards poverty, injustice, class divisions, the use of vernacular and vernacular lore and humour, and above all, the familiarity with rural life."

I agree that's exactly what you'd expect to find in folk song if it was wholly the product of rural communities. The problem is that as stated earlier, it's not clear that that is what we *actually* find in English folk song. How many rural folk songs are there that deal directly with poverty or class divisions, or which exhibit any familiarity with rural life beyond that which would have been universal at the time? Some, no doubt, like the poaching songs -- but they are dwarfed in number by the ones in which someone walks out on a May morning into a vaguely described idyll full of singing birds and skipping lambs.