I lived in Golders Green (Zone 3), and going to/from the tube station I always encountered few people. Since most of the sidelane was made up of single-family houses, it´s not so surprising.
Also, I had the ginormous Hampsted Heath park at a 10 minutes walk, parts of which are quite empty all day long.
Unlike many European cities, where the vast majority of buildings are medium/high rise apartments, many boroughs of London have parts more similar to American suburbs.
I certainly miss having a backyard.
London isn't all that dense, and the density is fairly unevenly distributed. There are forest areas within the boundaries of the city. There's a lake near my house - in the middle of suburbia, where people go fishing. I'm 10 minutes on a tram away from farms.
When I lived at Marble Arch and worked in Holborn (find "Oxford Street" on a London map - one of the busiest shopping streets in the world - and they're pretty much opposite ends) there was people everywhere most of the time, but even then finding quiet spots in the nearby parks, or by walking the back streets, was never hard.