What Vivaldi says is correct, although I'm not sure it is legal to insist on originals for music that is no longer protected by copyright law. However, these examining boards have the power to simply refuse to mark the exam, at least until they acknowledge the fact that it is legal to photocopy public domain music. And when they are protecting their own publishing businesses, that doesn't seem likely in the near future.
On their side, though is the argument that every examiner would have to become somewhat of an expert on what public domain editions exist for each item on the lists, kind of like a Carolus-type person. I don't think it's that difficult, but it does add an extra dimension of admin to the examiners' jobs, and it is much simpler to say they won't allow any photocopies.
I still get stuck, though, on the fact that photocopying public domain music isn't illegal, so it can't legally be prohibited . . . . .
In any case, most of these exams have a 20th century list, in the case of ABRSM, the B list. You might find early 20th century music on that list that is public domain, but most of what is on those lists will still be under copyright.
For the rest, you can send them an email and ask about public domain editions, but I don't think you'll get very far.
Interestingly (and annoyingly) enough, Trinity gave us a hard time a couple of years ago because they didn't want anyone even using library copies that were originals. One examiner maintained that each child had to have his own copy (increases their publishing income, although she wouldn't say that was the reason). I did take it up with the Head of Trinity when he came here on a visit. We do a lot of work with disadvantaged communities, and requiring each child to purchase 3 or 4 books a year is out of the question. He maintained that his examiners were told to allow library copies, that he thought he knew which examiner it was, and that he would speak to her. Guess what? The next year we were given a hard time about library copies again.
For this reason alone, I use ABRSM more than Trinity, and Trinity only when a student doesn't have the theory requirement for ABRSM. It seems to be only a matter of time before Trinity disqualifies someone for using a library copy.
Kind of like saying that if you want to read a novel, you can't read the library's copy . . . . .
