I think in all my 20 odd driving years I have only had cause to put my fog lights on twice, in both cases the fog was so bad I couldn't see more than 100 yards (think metres youngsters) in front of me.
The highway code lays out the law as to when you should use fog lights, it's perfectly clear.
Highway Code : Driving in adverse weather conditions.
Especially of note is this section.
236
You MUST NOT use front or rear fog lights unless visibility is seriously reduced (see Rule 226) as they dazzle other road users and can obscure your brake lights. You MUST switch them off when visibility improves.
This old road safety film featuring former Blue Peter presenter Peter Purves explains in more detail why using fog lights in rain is a bad idea, perhaps it should be re-broadcast?
OK, Grump over, carry on!