"These things work, in my opinion, when they take place in unusual locations at times where you wouldn't expect them."

Yes. Anyone attempting a musical Flash Mob is going to find it harder to get the surprise factor they are noted for, precisely because people are used to musicians busking in public places.

The difficulty would be to find locations where people won't be used to hearing buskers, and at the same time succeeding in not being thrown out.

A city's major graveyard (like Highgate Cemetery) might work because of the tourists. And (presumably) you're not going to get turfed out too sharpish. I did wonder if the turbine tower in Tate Modern might work, but I expect numbers of officious little people would bundle you off before you could so much as strap on your instruments.