Plenty of comments rolling in so let's have a pootle through the toot(le) issue...
Patrick9-32 reckons, "a lot of drivers are unaware of just how insanely loud cars are. Anyone who doesn't suffer from significant hearing loss and who is without noise-cancelling earphones knows you are coming from behind them, how far you are and when you start to overtake and how aggressively you are doing so by the sound alone, we don't need to look round, we don't need you to toot. In the words of Not Just Bikes on YouTube, cities aren't loud, cars are loud."
hawkinspeter commented: "The problem with car horns is that they have to be very loud to be heard above traffic noise by people inside metal boxes and this makes them quite startling if used behind you when you're cycling. Also, the main (only) purpose of a horn is to warn another road user that you are there (e.g. if a vehicle is reversing towards your vehicle having not seen you), so there is no point using a horn if you are following a cyclist unless they are about to turn across your path."
BalladOfStruth agreed: "You're not adding anything valuable by sounding the horn when passing a cyclist, you're just asking for aggro and risking a crash."
"I think part of the issue here is we are all used to drivers being aggressive to us," tigersnapper said. "I have had a few that seem to be aware we are vulnerable recently and being, if anything, overly courteous. I think we might need to accept there are some drivers out there who mean well, even if they are a bit misguided."
Sriracha made an interesting comparison between the horn scenario and ringing a bell at a pedestrian when cycling on a shared-use path...
"I appreciate the difference in scale, but I'd imagine a similar scenario plays out between cyclists and pedestrians when the cyclist uses their bell right behind the pedestrian, or yells out "on your right!'"
Fishpastesarnie replied: "Exactly my thoughts. I have certainly had more abuse using a bell than just shouting 'excuse me'."
Although GMBasix reckons, "It's the difference in scale that makes the difference. This isn't about cyclists racing past pedestrians on a shared path — that would be like a close pass from a driver, which isn't described here. A bell is not like being honked at.
"The cyclist knew the car was there, but the aggression of the horn appears to be his trigger. A bell is just not aggressive. Calling out is not aggressive. The equivalence you're describing is associated with passing the pedestrians.
"There is also another factor. While it is inconceivable that the cyclist was unaware of the car behind, it is not uncommon to encounter single, or groups of, pedestrians oblivious to their surroundings. Regardless of their place in the hierarchy, they retain a responsibility (as all road users do) to be aware of their surroundings and others using the road. In most instances, even a cyclist racing past should not come as a surprise. So a call out or a bell should not make them jump in the same way an unnecessary honk might."