Not many people realize that London is the world's greenest capital. Over 1/3 of the city is green park land. Parks are EVERYWHERE. There are the really large and famous ones, like Hyde Park or St James Park, and then there are the smaller obscure ones strewn about on nearly every block in the city. Most residential areas (like tower blocks, etc.) all have a shared park for its tenants. All of the parks are especially nice in the spring with all the flowers and trees in bloom, the warm weather, and for just sight-seeing and watching all the people, children and dogs.
On my normal walk from Old Street station to work every day, I pass through 3 such parks. This is a photo of King's Square Garden (and St Clement Church) early in the morning.
One of my favorite London parks is Hampstead Heath, probably the largest park in London. It covers nearly 800 acres and sits just 4 miles north of the city (just north of Islington where Paul's dad lives). From Parliament Hill (the highest point in Hampstead Heath), you can get spectacular views of the city. It's wonderful.