Here are the regs
http://www.opsi.gov.uk/si/si1989/Uk...iii)25requirementaboutuseheadlampfrontfoglamp
The salient parts being ,
"Requirements about the use of headlamps and front fog lamps
25.—(1) Save as provided in paragraph (2), 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-
(a) during the hours of darkness, except on a road which is a restricted road for the purposes of section 81 of the Road Traffic Regulation Act 1984 by virtue of a system of street lighting when it is lit; and
(b) in seriously reduced visibility.
(2) The provisions of paragraph (1) do not apply-
(a) in the case of a motor vehicle fitted with one obligatory dipped-beam headlamp or a solo motor bicycle or motor bicycle combination fitted with a pair of obligatory dipped-beam headlamps, if a main-beam headlamp or a front fog lamp is kept lit;
(b) in the case of a motor vehicle, other than a solo motor bicycle or motor bicycle combination, fitted with a pair of obligatory dipped-beam headlamps, if-
(i) a pair of main-beam headlamps is kept lit; or
(ii) in seriously reduced visibility, a pair of front fog lamps which is so fitted that the outermost part of the illuminated area of each lamp in the pair is not more than 400 mm from the outer edge of the vehicle is kept lit;
(c) to a vehicle being drawn by another vehicle;
(d) to a vehicle while being used to propel a snow plough; or
(e) to a vehicle which is parked."
and ...
"(1) (2) (3)
Item No. Type of lamp, hazard warning signal device or warning beacon Manner of use prohibited
1 Headlamp
(a) Used so as to cause undue dazzle or discomfort to other persons using the road.
(b) Used so as to be lit when a vehicle is parked.
2 Front fog lamp
(a) Used so as to cause undue dazzle or discomfort to other persons using the road.
(b) Used so as to be lit at any time other than in conditions of seriously reduced visibility.
(c) Used so as to be lit when a vehicle is parked.
3 Rear fog lamp
(a) Used so as to cause undue dazzle or discomfort to the driver of a following vehicle.
(b) Used so as to be lit at any time other than in conditions of seriously reduced visibility.
(c) Save in the case of an emergency vehicle, used so as to be lit when a vehicle is parked."