Taken from http://www.opsi.gov.uk/si/si1989/Uksi_1 ... ningbeaconWarning beacon emitting blue light and special warning lamp
Used so as to be lit except-
(i) at the scene of an emergency; or
(ii) when it is necessary or desirable either to indicate to persons using the road the urgency of the purpose for which the vehicle is being used, or to warn persons of the presence of the vehicle or a hazard on the road.
7 Warning beacon emitting amber light
Used so as to be lit except-
(i) at the scene of an emergency;
(ii) when it is necessary or desirable to warn persons of the presence of the vehicle; and
(iii) in the case of a breakdown vehicle, while it is being used in connection with, and in the immediate vicinity of, an accident or breakdown, or while it is being used to draw a broken-down vehicle.
8 Warning beacon emitting green light Used so as to be lit except whilst occupied by a medical practitioner registered by the General Medical Council (whether with full, provisional or limited registration) and used for the purposes of an emergency.
For a little light reading the full regs here http://www.opsi.gov.uk/si/si1989/Uksi_19891796_en_1.htm
2004 amendments (.pdf) http://www.dft.gov.uk/stellent/groups/d ... lations%22
2005 amendments http://www.opsi.gov.uk/si/si2005/20052559.htm