This is what happens when you search for "London Parks" in Google:
As you can see, fortunately, Central London has a very high density of parks, and they come in different sizes and with diverse landscapes and history. Starting with the Royal Parks, we can't fail to mention Hyde Park, St. James's Park, The Green Park, The Regent's Park and Kensington Gardens, each with their unique class, history and design. Hyde Park, for instance, covers over 350 acres (the equivalent of 4,550 tennis courts!) and features very diverse flora and fauna, from great crested grebes to bats and the tons of diverse insects and invertebrates the wildfowl feed on.
When I go for a walk, though, I prefer the smaller, less visited squares, with their regular pigeons & squirrels and placid greenery. Fortunately, London is packed with these little wonders. Oases of peace, where time seems to move slower, and the air invites to calm contemplation. St. Anne's Church Gardens, in Soho; St. James's Square in St James's (below) or Tavistock Square, in Bloomsbury quickly spring to mind as perfect examples.
One thing I love about the British weather is that it nurtures the ground and allows these amazing spaces to exist, because London would be very different without its parks!