I'm not suggesting Jack sings in that awful fake "English" accent. I am saying that the people on the English folk scene won't book an Englishman singing in a fake American accent. That's how it is. (not necessarily how it should be).
The folkie you quote made his money, as you say, by abiding by the rules. The mavericks who didn't often had a greater talent but got less work.
The point is, if your face doesn't fit there's nothing you can do but compromise, or find a way to retain your enthusiasm and carve out your niche.
If Jack is unhappy with a small room of people who obviously know every word of his songs then that's a shame. If singing and writing brings him feelings of frustration and bitterness, then for his own self-preservation he should stop doing it.
The only attitude that works (and pardon the football analogy) is to realise that you're not wanted by the Man Utds even though you can't understand why, but work hard at becoming a legend for Hull or Walsall. If you love playing, that can be more than enough.
And I know some very fulfilled passionate artists who've done just that, and I hope Jack can rise above his frustration and find peace in his talent.