Len, yesterday:

Whether it's day or night, if the visibility is seriously reduced by, let's say - fog - then you are by law allowed to turn on your foglights, provided they will not dazzle other road users - the visibility restricting effect of fog prevents other road users being dazzled unless, as flagged by Guy - they are very close to you.
It's no good quoting 'the Law' on this, because 'the Law' is the interpretation of The Road Vehicles Lighting Regulations 1989 Act. The interpretation of this act by everyone but Len seems to be as follows:
If it's dark and foggy thus visibility is seriously reduced you should turn on rear and/or front fog lights to make other road users aware of your position. Rear fog lights allow your vehicle to be identified from a greater distance in fog, thus improving safety.
If it's dark and the fog clears, you should turn off your foglights, as they will dazzle people.
Len, do you use the middle lane of motorways like, a
lot?