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.
The City is a specific area, more or less covering the same area as the original Roman city. It's the original financial district - though a lot of that moved to Docklands at the tail end of the 1900s.
It's much more built up than even adjacent Westminster ("The City of Westminster") and definitely has far fewer 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.
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 of an urban environment so much.
I would say they are pretty well distributed through places where people actually tend to live. I live in a pretty average residential area in zone 3 and not only are there nice parks nearby but there are plenty of trees. London is of course massive so I can't say it's the same everywhere but most residential areas I've visited have been quite green. The City and West End (very much commercial/touristy areas) are the exception in my experience.
My guess would be that the bio-diversity net gain calculations put the ecological investment off-site where it was more practical.
It's a shame though as trees and architecture can happily co-exist with each other. Living walls and well kept green areas are entirely possible.
Clicking once into Canon street towards those trees presents me with the trees. They're now in leaf and look like Sorbus intermedia "swedish whitebeam" and the key id is the margin on the leaf and the green fruits. Photo was taken July/August as prior to that they're in the flowering phase (beautiful to see btw).
When I spin the view down Canon street I see three mature trees in full leaf on pavements / sidewalks.
As I said in another reply, I'm an arborist and I'm hardwired to see trees and perhaps I subconsciously avoid areas that have none, so maybe that's bias on my part.
Here's a map of the canopy data.
Elsewhere though, possible to plan continuous walks through greenish spaces. One starting at Victoria: Belgravia back streets, Hyde Park, Grosvenor Square, Marylebone High Street, Regents Park, Primrose Hill, Belsize Park, Hampstead Heath.
It gets greener as you go further out.
One of the big problems in the UK has been the rise of low maintenance gardens, replacing plants with decking concrete, gravel etc.
It makes me wonder whether they know which bit is actually the City.
later on:
> Sure, you have a small/big park here and there
What big park is there within the City? The whole of the City is smaller than Hyde Park (including Kensington Gardens).
Same shock, different direction, much nicer.
The City is indeed pretty non green
Personally I have always felt that most Japanese cities are very devoid of urban greenery compared to UK towns and cities.