Now maintained by English Heritage, Down House is currently subject to a bid to become a registered World Heritage Site and is maintained as a museum to Darwin's work.
Entrance is a rather steep £8.80, and it is probably best accessed by bike (perhaps via train to Orpington - 25 mins - though a bus is an alternative option), the house is around 15 miles from Central London, surrounded by countryside and near Biggin Hill airport. If you want to make a day of it, there is a tea room at the house, or the nearby village of Downe has two pubs.
For more information visit the page on the English Heritage website at here.
Click here to view the location on a map.