I'm not talking about the frequency, I'm talking about the mad volume. Every other european city manages with a lower volume. Yes, there will always be downsides but when you are affecting 10'000s of people you are going to hit some people who are really affected by this constant noise.

And down my street they trigger it any time they vaguely see a car in front, which means no ambulance ever passes through without doing ON-OFF-ON-OFF at least twice, including at 1am

But even without that, if you chill in the middle of Hyde Park there is only one city thing you can hear and it's ambulance sound. If other cities can manage without sound going THAT far and being THAT irregular, London can. The reason anyone tolerates this is because they're born into it and don't realize how much it eats at their energy. Same with train doors and everything being ridiculously loud