Here are a couple of observations (not pronouncements!) about harmony singing in Britain in earlier centuries.

I have heard that to some extent it was a regional thing. In some regions, you sang harmonies, in others no. In some areas, only the lead singer sang refrains and choruses -- They didn't expect to have any audience participation at all. This tradition was carried on, for example, in Sodom Laurel, NC, where Sheila Kaye Adams was raised. I believe Norman Kennedy said the tradition where he came from was to sing along on choruses, but in unison. If I'm wrong I apologize. I do know he said, categorically, that he prefers no harmony singing on his choruses. Lou Killen, on the other hand, said that where he comes from they certainly do sing harmonies.

Maybe there's also a difference in the type of songs from region so region. Love songs and ballads are one thing but livelier songs may have been treated differently. It's hard to imagine that a harvest song in the pub, or accompanying the last hay wagon home, had no harmonies! In fact as I think of it, I suspect the songs that invite the best harmonies mostly came from regions where harmony singing was prevalent. These are all suggestions & theories folks, just throwing out ideas for discussion (or not).

About shanties, I read something once that made me suspect they were actually frowned on in some quarters. How much difference that made to the seamen is another question. What I read was a quote from a British writer who had heard a crew singing shanties and said it was all well sung, except for one problem -- there were black sailors from the West Indies in the crew, and they "could not be taught to stay on the tune." !!!!!!! It makes me sigh, both at the arrogance of it, and at the ignorance. Surely the West Indian sailors were perfectly capable of singing the tune but sang harmonies because it was what they liked. I have wondered how much their singing encouraged other sailors to harmonize.