I found the following paragraph taken from the latest edition of Motorcycle Roadcraft
Observing when visibility is low.Always be prepared for a sudden stop in the traffic ahead. Don't follow closely, and only overtake other traffic when you can see that it is absolutely safe to do so. This is seldom possible in fog on a two-way road. At junctions when visibility is low, wind down your window and listen for other vehicles, and consider using your horn.