The venue should be paying a licence fee to the PRS but if they don't get to know what is performed, and if most of the composers/arrangers aren't members, then little if any of this will find its way back.

The PRS system is based on a much more structured and professional music scene, and as you rightly say it doesn't serve the informal and largely non-professional folk scene very well.

More formal performances, especially festivals, do make returns and folk musicians who are PRS members do get paid.