In th 17th century, a well was dug in Streatham at the spot that's now occupied by "The Rookery" community garden. Streatham Common is next to the Rookery, and comprises woodland at the eastern end, up the hill, and mown grass rolling down the hill westwards. Standing on top of the hill in front of the woods, you can face west south west looking across to Tooting, Wimbledon and Kingston. If you look carefully, you might be able to make out Hampton Court Park.
At the end of the 19th century, the Common was bought by The Metropolitan Board of Works, preserving it as a public space. At the start of the 20th century, the building in the Rookey was demolished and the gardens were opened to the public.
While you're here, have a walk around the Rookery gardens, or else see if you can find the "Lightning Tree" on Streatham Common - it's still alive, but seems to have been burnt out somehow, so that you can actually climb into it.