As I understand it, in law, material is either in copyright or out of copyright. If a work is credited as Trad or Anon, it's assumed to be out of copyright - by PRS, MCPS, equivalent organisations overseas, and everyone else, and treated accordingly. Sometimes, just sometimes, someone may know better and then the tag may be rectified.

Again, as I understand it, PRS/MCPS is not a pressure group. It has a mandate from government to administer royalties in the UK, so in common law that makes their definition the legal one. But I'm willing to be proved wrong on that.

In any event, if the dichotomy was cleared up, the legal problem would disappear.