I would have gone for Hampstead Heath next so your go Mr Nines.
I forgot about Hampstead and Crystal Palace park too. An interesting geographical observation is that if you look at a map of London most of its main parks are all in alignment. So from North to South it would be Hampstead Heath, Primrose Hill, Regents Park, Hyde Park/Kensington Gardens, Green Park, St James's Park, Battersea Park, Clapham Common and Tooting Bec Common.
70% of our planet is covered by water. What percentage of Earth's water is drinkable?