A singing protest would only need a licence of it was organised!
If it "just happened" then on licence required.
Of course it can't "just happen" if you advertise it, but if a lot of like minded people are in the same place at the same time then a sing may break out.

However this is just the begining of mass closure of village pubs, when the smoking ban bites we will see hundreds.

MM I'm ready for a dress off when you are!
Skipy