and the UK City with the greatest number of trees per square mile is.................Leeds.What the hell are you on about? London is about as green as it gets - the parks are MAHOOSIVE, and every second square seems to be a little oasis of greeness where you Londons can sit and be unfriendly to each other, whilst drinking your over priced skinny crappy-chinos and Pret A Mung Bean Burger.
You want built up, come up North.
And, actually, we've never done public space well in London, which is why the loss of it is so sad. New York was planned around a massive park and small open squares every few blocks, but, by the time that New York got going, London's population was nudging two million and the game was effectively lost. There's lots of small parks in the 'burbs and lots of people in the 'burbs have gardens, but zones 1 and 2 is remarkably short of the green stuff, and that which we have is mostly Royal Park in the ritziest parts of town.