In practise, would someone just care to draw up a list of all-English folk songs we could choose from? Who is going to decide?

In practice, anyone who wanted to decide would decide - and anyone who wanted to object to those people's decision could object to it. It's only in theory that there's any need to draw up a Definitive List of Definitively English songs - and since gigs don't happen in theory, I don't think it's a big problem.

I think we're answering different questions. Your question is "should we, as anti-fascist folkies, exclude non-English songs on principle?" and obviously the answer to that is No. But mine is "should we, as anti-fascist folkies, try to include non-English songs on principle?" - and I think the answer to that is No too.