I used to commute to work by bike in ~1M city in Europe, mostly on dedicated bike lanes, but some shared, and had just the smallest, barely audible bell, only because it was required by law. I don't remember using it much at all. I don't know what the problem is. Maybe the Londoners should take a good look at themselves.
I agree that on a footpath pedestrians should be treated as having priority.
A semi-common way I use my bell: when on a shared footpath with plenty of space to take over, I often use my bell when I'm still ten meters away, so that I don't give pedestrians are heart attack by suddenly dashing right past them.
(I have a nice ding dong bell. They don't seem to mind. It also helps that I often have a cheerful five year old in the back.)
For example, I can go into datacenter and it will cancel all the datacenter noise(aside for when air blows directly into mic, it overdrives it) but I can still hear what other person is saying.
Also I used them to generally listen to podcast so there was no wall of music to go thru, so sirens and such were easily discernable
If you listen carefully you can usually hear a cyclist behind you who may want to pass or is passing you, and having headphones probably makes that a lot harder
The most problematic people in traffic are never aware that they are the problem.