I pass nearby most mornings on my way to work, and have lived in this corner of London for over a decade... but I never knew it was there!
It is a real working farm, tucked away in a residential corner of Deptford/Bermondsey/Rotherhithe. You can walk up to the Thames Walk nearby, and from some spots you can even see Canary Wharf!
You can read more about it over at the Occasional Tourist.