As Mamya says,
" . . . no person shall use, or cause or permit to be used, on a road a vehicle which is fitted with obligatory dipped-beam headlamps unless every such lamp is kept lit . . . in seriously reduced visibility"
[s25(1), Road Vehicles Lighting Regulations 1989]
A large part of the problem is auto headlights. They don't come on in fog etc and people just assume they do. Too many automatic things on the cars of today. You can still be prosecuted for incorrect use of lights or not using them when conditions dictate.