I'm not sure that the "arrangements" vs. "composition" argument is very strong. To some extent RVW made an arrangement, and Bellowhead are composing stuff around traditional material.

To me the big and much clearer difference is that with RVW, the composer sets in stone every note that is to be played, and the orchestra plays no part in that creative process. Many different orchestras will play Vaughan Williams' work, using the same instruments playing the same notes.

On the other hand, Bellowhead's arrangement is for Bellowhead alone to play, and if another band want to do a version of the same song, they will most likely have different instruments and make a quite different version of it.

In other words, in folk music the performers have huge creative freedom over the details of the music, whereas in classical music they do not. That's the point LTS was making about her choirs, and also one of the reasons why I like doing folk music (and I've played in orchestras, string quartets etc. as well so I know what that's like)

Anahata