I think in UK that 'begging' is covered by the Vagrancy Act. In begging, under the act, the approach is for money and no service is offered. In the case of busking the service is provided gratis. The public can choose to engage with the service or to dismiss it. There is no confrontation, no threat and the risk is all on the part of the performer. The performer must NOT ask for, or request recompense. Definitely not begging.