KT, I have never had the privilege (yet!) of being at a pub session in the English part of the UK, but have been to a few in Scotland, and have also been to many in Ireland, mostly in the West. Many are basically instrumental sessions, but many do in fact revolve not around a particular singer or performance, but around the sharing of songs around the room. It happens here, too, if you are in the right place at the right time.
At what could possibly be my all-time favorite, in a small place I am almost loath to identify, participants ranged from about 12 to over 80. When a song with a chorus came up, the roof was raised well and proper. When a solo type song came up, one or two men would (gently but firmly) say to the joiners-in, "one voice, please." (A tradition I have wished for here on occasion.) And possibly my favorite custom there was that when the ambient noise level had crept up between songs and someone started a song in a soft voice (including an elderly lady who was very quiet but new astounding songs) someone with a more robust voice would say, "Song here!" and people would fall quiet to listen. At one point, too, he said "Song from an ELDER here!" and you could have heard a pin drop, though I do not know how that would fly in the U.S.