My suspicion is that the OP is thinking largely of modern groups. (By 'modern' I mean anything really from the last 40 years) If you think of recordings of English groups, it's hard to think of any group that doesn't arrange their material in thoroughly harmonised form (or what a jazzer would call reharmonized or reharm versions).

As some have pointed out above, there are certain instruments that harmonise by default - eg pipes with their drone. While this unquestionably a type of harmony, there's still a world of difference between a piper and a fiddler duetting in an Irish pub recorded on an old Topic album for example and, say, what Leveret or English Acoustic Collective do. I'd love it if more English musicians made 'pure drop' albums.

For instance, I've heard both Sam Sweeney and John Dipper playing entirely solo sets live, and in both cases I thought it was way more engaging and subtler yet also more intense than in their group recordings.