http://www.guardian.co.uk/music/musicblog/2010/aug/31/transport-for-london-buskers

One thing that may shed some light on all this is another list which definitely does exist and has been distributed to all buskers. This one concerns specific instruments banned on specific pitches. It includes items lsuch as "robotic sax" – which appears to be there specifically to ban a guy called Steve Aruni, who plays guitar along with a latop-controlled modified sax-playing Henry the Hoover. Genuine health and safety concerns may lie behind some of these specific pitch bans; others seem totally arbitrary.