Horns don't work in California. They don't work for cars, they certainly don't work for bikes. I've watched a car ruthlessly cut off another, and the guy getting cut off laid onto the horn for probably a solid 10 seconds. No one-- especially the car doing the cutting-- made any reaction whatsoever. It was just another loud noise in a sea of loud noises. You could ride down the street with a car alarm going off, and still get close-passed. The only thing that would get you properly noticed, and probably arrested, would be the ability to generate that woop-woop siren quick-burst cop cars can make. That gets a reaction out of people.

I've just taken to yelling Oi! at the top of my lungs at cars that are behaving badly. People here don't know what to make of Oi! so they actually pause to see where it came from. Probably less effective in the UK. Also, free, no batteries required.