They are illegal under the road vehicle lighting regs!
Restrictions on the use of lamps other than those to which regulation 24 refers
27. No person shall use, or cause or permit to be used, on a road any vehicle on which any lamp, hazard warning signal device or warning beacon of a type specified in an item in column 2 of the Table below is used in a manner specified in that item in column 3.
(i) to warn persons using the road of a temporary obstruction when the vehicle is at rest; or
(ii) on a motorway or unrestricted dual-carriageway, to warn following drivers of a need to slow down due to a temporary obstruction ahead; or
(iii) in the case of a bus, to summon assistance for the driver or any person acting as a conductor or inspector on the vehicle.