If they had already been paid, then no, it isn't acceptable to demand money. It's possible that a hard nosed copper could see it as demanding money with menaces and nick them for it. They could certainly be nicked for begging, particularly as they have already received a payment.It's like having a service charge and expecting a gratuity. They have been paid once, if it wasn't enough they shouldn't have taken the gig.
If busking is their sole income, then yes, they have a right to be reasonably firm in the way they request payment. If they have another set income (payment for the gig) then they have no right to forcibly demand more. And I bet they don't declare the pickings to the Inland Revenue.....
LTS