Pet Cemetery - Kensington Gardens, W2
Now this is a quirky one. Its not really lost or derelict its just hidden away in someone's back garden so not accessible to the general public.
In 1881 the keeper of Victoria Lodge buried a local family's pet dog (who loved walking in the park) in his back garden. The idea caught on with other locals and very soon it unofficially became a cemetery often for dogs who came to their end after being trampled by horses and their carriages in Hyde Park such as a terrier belonging to the wife of HRH Prince George, Duke of Cambridge.
There are over 300 little graves here - mainly dogs but a few cats, birds and a monkey.The cemetery is mentioned in the Little White Bird novel by J. M. Barrie and "The Optimists" with Peter Sellers had scenes filmed here. George Orwell is reputedly to have described the cemetery as perhaps the most horrible spectacle in Britain.