This from answers.com: "From the early 19th century, legislation that was applicable only to the London metropolis used Charing Cross as a central point to define its geographical scope. Its later use in legislation waned in favour of providing a schedule of local government areas and became mostly obsolete with the official creation of Greater London in 1965."
definitely charing cross - there is a stone obelisk outside the station - the history is documented - it is from this that distances to and from london are measured