There is an absence of lawn maintenance machinery sounds.
It is true that there is a lot of green space in London, maybe even more green space than any other major city in the world but even with the vast amount of green space, there is an absence of lawn maintenance machinery sounds like lawn mowers. I never hear lawn mowers unless I happen to be in a park or square when lawn maintenance is taking place and the chances of that happening are incredibly low.
It is also true that homes and flats have gardens but gardens generally do not have grass. Instead, they are paved with some sort of stone or decking or maybe even green carpet like our landlord did to our back garden after we moved out of our flat in Chiswick.
What sounds fill the absence of lawn maintenance machinery? Diesel cars, motorbikes, metal scaffolding being installed on buildings, building construction, dogs barking, kids screaming, police/ambulance sirens, helicopters, trains, and airplanes.