The capital contains 35,000 acres of public parks, woodlands and gardens, which means that 40% of its surface area is made up of publicly accessible green space.
In comparison, Berlin, the next major city green space provider in Europe, has just 14.4% of green surface area.
Boris reckons London's the Greenest City on Earth. Looks like it's the greenest in Europe, third greenest in the world:
http://www.edie.net/news/6/London--greenest-city--in-Europe-/