Having moved in London two years ago, I struggle to find a moment of intense quiet. The reason is mainly the planes that are authorized to fly above the city (a stupidity in my opinion) and even in the middle of a park, you can still hear them, flying just above your head.
At night, cars, ambulances, you name it, will take the relay.. Frustrating.
About 10 years ago I was living in the UK a few miles from Heathrow and near a motorway. I was working late shifts so I'd be walking home anywhere between 3 and 5 am. Once or twice in that couple of years I noticed it was totally quiet for up to a minute at a time. That was it: maybe two times in a a year!
(I also often got stopped by the police, who wanted to know what I was doing _walking_ at that time of the morning. But that's a different story...)
Coming back to NZ was a joy.
That's still something I recall from the days immediately following 9/11.
No airplanes overhead.
Occasional fighter overflights excepted. But where normally every 60 seconds or so a commercial transport (or two) would be heard ... nothing.
This actually impacted weather patterns too -- cloud cover notably diminished as was reported at the time (subsequent studies dispute some of the findings):
It really depends where in London you are as well. Where I am (N London, 25 mins on the Tube from Moorgate or the west end, so not exactly far from town) it's quiet enough that I can hear owls at night and only the occasional car or plane.
At night, cars, ambulances, you name it, will take the relay.. Frustrating.