The Singing School movement of the 18th century was trans-Atlantic. One of its principle aims was to have everyone singing 'by note' not 'by rote.' It was mostly for church music, but there are secular works that pop up, such as Billings' Chester. There was also a felt need (whether justified or not) for more tunes. The music was almost always written in 3 or 4 parts.

In UK the music of this tradition is often called West Gallery. There was surely some mutual influence between it & the pub carol singing traditions. It seems to have become choir (quire) dominated fairly quickly.

In the US the New England tunesmiths gave rise to the Shape Note traditions that have kept it out of the hands of the choir. The 'all day singing' of the 7 shape Gospel Songster, Stamps/Baxter style did become more oriented toward quartets, but more in the 20th century.

The harmonic structure of the 18th century music fell out of favor under the influence of German music theory.

In the US, the practice of everyone in church using a hymnal with a close score in 4 parts, means that anyone can start trying harmonies out if they so desire. I noticed in England, that most churches use hymn books without music. This means only your choirs get any guidance in how the harmonies go. There are sectors of thought in regard to hymns that the singing should be in unison. I think Vaughn Williams may have advocated that practice. There are often organ accompaniments that are deliberatly written to obstruct harmony singing.

Hrothgar, where are you? I might be able to point out some all day singing. There's nothing quite like singing 80-90 tunes in a day.