This morning, I found myself in the unusual position of being stuck in a queue at the traffic lights behind a a van (a white van, no less) whose driver seemed not to have noticed that the lights had changed. At last! A chance to use my bike horn, if not in anger or revenge for being on the receiving end, then at least with a typical Londoner's impatience.
I could almost say I followed this advice from a Japanese car hire firm, as quoted in Charlie Croker's "Lost in Translation" (the perfect loo-book):
When passenger of foot heave in sight, tootle the horn. Trumpet him melodiously at first, but if he still obstacles your passage then tootle him with vigor.