I managed to find
Highlights of the Mayor’s Ambient Noise Strategy from 2004 which stated that:
Helicopter noise can be intrusive and irritating. The Mayor does not have any powers over movements. As long as helicopters obey the Rules of the Air, and follow instructions of air traffic controllers, their movements are not generally further restricted. Much helicopter activity over London is for security and emergency purposes. When used for such purposes, helicopters can fly lower than rules would normally require.
RC