You MUST use headlights when visibility is seriously reduced, generally when you cannot see for more than 100 metres (328 feet). You may also use front or rear fog lights but you MUST switch them off when visibility improves (see Rule 236). Law RVLR regs 25 & 27
Darren has owned several companies in the automotive, advertising and education industries. He has run driving theory educational websites since 2010.