"Almost all the songs mentioned here were sung (and revived) by solo singers. No-one seems inclined to discuss wassail songs, work songs, local carols, drinking songs etc."

I'm not sure whether your 'no-one' refers to this thread or to the collectors, Guest jag, but Cecil Sharp for one collected dozens of carols, wassails, shanties and a good few songs about ale, and also lectured on the first two. Lloyd talked a lot about them as well, and I think most people regard them as an essential part of the 'folk' canon. Baring-Gould collected a lot more in pubs than the others, and heard more communal singing, also a few specific instances of two-part harmony.

None of that changes my view that English traditional singing is primarily unaccompanied, but pub singing does need to be taken into account - for example, Cyril Poacher appears to have added a refrain to 'The Broomfield Wager' only when he sang it in the pub.

"From the descriptions of and by many collectors there seems to have been a collection bias towards these features [modal and gapped scales] and a tendency of source singers to offer them."

I'd say more a publication bias than a collection bias, though of course we'll never know for certain.