I'm not sure what parts of London you were in, but there's many trees in London. There's even a specific species for it - the London plane (Platanus × hispanica)
If you're in the very new, constantly rebuilt, concrete jungle that is the very small part of the city, then OK, greenery is going to be hard to spot. Particularly as they tend to nearly always choose the wrong species to plant and aftercare is an afterthought. But your assessment is factually incorrect.
See for yourself. Go to Google maps, drop a good few street view randomly around the city and you'll see that more often than not you'll see trees.
Also, I have a networks in arboriculture who work in the city and they're never short on work.
I'm not doubting your experience of unease or a concrete/glass wasteland (that's yours and not mine to question) but the facts don't support the statement of no random trees on pavements (side walks).
I live in the North, but I'm often in London.
I'd echo the gp's thoughts. There are parts of the City and the West End that are basically devoid of trees.
My biggest bugbear in London is the number of developments that have a "token tree" with one lonely tree in one corner, often doing quite poorly, presumably included to check some item on a planning consent checklist.
Of course, London has many green spaces and on the whole has plenty of trees, it's just they're quite unevenly distributed.
Maybe I'm just in different places. Normally I'm walking from Kings Cross down Grey street and around Covent Garden type areas.
I'm nearly always on foot. Perhaps it's just because I'm also an arborist and I'm hard wired to see trees and avoid places that don't have them?
The token tree thing is a problem. Daisy Barrington was part of webinar on the topic as part of the Arboricultural Associations webinar series [0]. Rarely do the species planted get based on local ecology and or have a solid aftercare plan. They're normally chosen for immediate aesthetic look (Paper / Himalayan birch being the most common) rather than how they'd exist over time.
In short birch being a pioneer species is short lived (80 years), grows fast towards light and dislikes being pruned. Where as oaks, norway maple, London planes ( some of which are "climax species") etc live for longer, grow slower and respond to pruning better, support local ecology better and some don't mind the pollution the an urban environment so much.
[0]: https://youtu.be/Kql22dZlq6o?t=2407
open Google maps at Monument station, find a tree in the area. all the streets in that region of London (let's say 1 sq km) are quite narrow, I would guess there just is not enough space for street trees.
What? London is one of the greenest cities in the world.
I'm talking about trees on sidewalks and streets, not about parks.
The city government tracks data on public realm trees, and has a nice map based visualization of it: https://www.london.gov.uk/programmes-and-strategies/environm... and if you zoom in you'll see that many of these are streetside trees.
Personally I have always felt that most Japanese cities are very devoid of urban greenery compared to UK towns and cities.