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.