I got my first chance to use the fog lights in the Skoda tonight, driving home from my rehearsal. It was patchy but, at times, visibility was definitely down to the point when they were needed… so just as well I knew how to activate them!
I double checked the definition when I got in and it turns out not to be straightforward. The Highway Code suggests you may find it helpful to use them when visibility drops to 100m or less, more or less, and also points out that you shouldn’t use them otherwise. The RAC website, where I started my search also mentions 100m but points to a Met Office page which gives the official definition as less than 1km and the general usage as less than 180m.
I think my rule of thumb – when you are glad other cars are using their lights or you wish they were – is probably good enough to cover the ambiguities, combined with the crucial point of not forgetting they are on when the fog thins out and you can see a good distance ahead.