Don't really know about this from a magic perspective, but from music. I shouldn't have thought the rules are that different apart from less of a worry with magicians making too much noise. This site has the regulations to download.
http://www.artscouncil.org.uk/publications/index.php
Can't post a direct link, but if you go on publications, then by title and on page 19 there is "A simple guide to licensing for circuses and street arts" which has the info needed.
As said it's different place to place. In London I imagine its regulated quite well in particular areas, whereas me in Hull I doubt there would be an issue. They'd be happy for a free entertainer on the street. Basic rules come down to not being in the way, blocking street, fire exits, actively request money, no pressuring people by shaking collection hat, etc. The shaking collection things is particularly disliked by some police authorities, with police waiting round corners to hear that jingle. Basically check with your local council for the license and apart from that have fun. People seem more keen to give you money when your enjoying yourself.