Would appreciate information as to how common joining in with the singer now is in the UK - and opinions thereon

As an occasional singer in clubs, I'm always happy for the audience to join in - if they know the words, which is sometimes highly unlikely, given the abstruse stuff I do! I've not seen or heard much evidence of thoughtless joining-in in the clubs that I frequent - audiences appear to be mostly tactful. If the performer indicates quite clearly that he/she wants audience participation, that's a different matter.

At the sessions I attend, it's understood that, unless the leader of a song or tune specifically wants to do a solo and says so, joining-in is the norm. At my own session, I always make sure newcomers understand this.