"It was also the case that there were probably around 50% more people than usual at our singround that clashed"

Our open mic was the exact opposite.

To be honest, your experience, rather than mine, was the one I had expected. It would be interesting, albeit almost impossible to achieve, to find out what happened more generally. Not only open mic sessions, of course, but cinema attendances, theatres, museum visits - my daughter works at the Natural History Museum and they have greatly extended the opening hours - and so on and so forth. As well as museums my daughter works in theatres a lot, and it seems the impact of the Olympics is The Great Unpredictable - no-one seems to confident whether attendances will be up because of more visitors doing things when whatever they specifically turned up for is not happening that day; or down because the usual audience for the theatre is staying at home or going to the venue.