And what about putting a notice about it on a listings page on the Internet?

And would sticking a notice up outside the pub count? I suspect it would.

What about a warning to customers? - "If you don't like folk music, stay away from the pub on such and such a date, because those people you don't like will be sitting the bar making what they call music".

They don't think these things through.

The trouble is, it isn't what the law actually says that matters, or even what the local council might think it says. It's what the people who own the pub - typically some big company these days in much of the country - might think it meant, and what their solicitors and accountants, might think was safest. And that is likely to be a policy of "no unofficial music".