Z'fafr as I know it's the driver's responsibility to ensure they work each and every time they go out. Before someone flames me for that, I'm not saying 'thou shalt check thy lights before every journey', just that if the poliss stop you and one doesn't work, unless you can categorically state that you checked them before setting off then you won't really have a leg to stand on.
One thing I can't understand, back in 1988-89 I had a 1982 Fiat Strada Super 85 with a warning light system built in. The dash had a little car schematic in the instrument binacle which showed a warning light if a lamp was out. Now, considering this was a used Italian car with the usual reliable eletrical system, this worked properly, and the two or three times I had a lamp fail it showed on the panel. Why can't modern cars have this facility?