Not to one-up you, but Sheffield in South Yorkshire has even more trees. I visited recently and was taken aback by how much coverage there was for a city center, coming from Leeds, which seems to hate trees.
If I remember correctly, London has around 8.5 million trees (for a population of almost 9 million, 1:1 tree:person) and Sheffield has around 4.5 million (for a much smaller population of less than 600,000, 7.5:1 tree:person). Sheffield has more trees per person than any other city in Europe.
Sheffield council tried to cut down thousands of trees a few years ago and it caused massive backlash, protests and arrests.
https://en.wikipedia.org/wiki/Sheffield_tree_felling_protest...