This is not being posted by Bill D, actually this is Ferrara, Bill's wife....
I'm very pleased to see evidence that harmony singing in British music has ancient roots. Good harmonies are so satisfying to sing and hear.
Here's another point of view though. Norman Kennedy gave a ballad workshop here recently and I asked him whether it was appropriate to sing harmonies on choruses, refrains, etc. He said in his opinion, absolutely not. In fact he said it will throw him off. When he learned old songs in Scotland, the group singing was all done in harmony. He said the current style of harmony singing developed in the context of "set pieces," ie performances I guess.
Possibly there are regional differences?