West gallery singing also includes harmonies. Once you learn to make harmonies in church, why would you not use them when singing secular songs?

The collectors often went round and collected individuals. With someone like Vaughan Williams, for instance, he was specifically interested in captering and preserving folk melodies, and not necessarily preserving the ways in which those songs were sung. He didn't often collect people in the natural environment of the pub, for instance, but alone, in their own homes, and he didn't record very much, but largely notated tunes. He did used to go to pubs and hear singers, but I don't think he really talks about whether there was any collective or harmony singing in these cases, or whether the singing was ever accompanied.

Yes, there is a strong tradition of solo, unaccompanied singing in England, but that doesn't mean that it was the ONLY tradition. Some of the living traditions we still have, for instance, would suggest otherwise. The Copper family sing harmonies, and their tradition goes back at least eight generations. Sheffield Carols, still sung each year in the village pubs, contain strong, spontaneous harmonies. And though I've never been to any shepherd's or hunt meets to hear any singing, I'd be interested to know whether people sing in harmony - I know that Will Noble and John Cocking, for instance, do.