I don't think the point is that London literally lacks pavement trees. As you say, the London plane is almost part of the city's visual identity in many areas. The interesting thing to me is how uneven the experience can be

Totally, "the city" (EC1/2/3/4) as the GP says is pretty barren, especially the newer built areas.

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.