If one wanted to park his car for free on the street and visit London's city center, what would be the closest/most accessible place to do so? For the sake of the question, [let's assume that the centre of London is Charing Cross, Westminster][1].

The parking spot has to be free of any kinds of restrictions (e.g. only free on certain days) and has to be available to non-residents. Obviously parking in Central London itself would never be free, but surely there is free parking somewhere within a 20km radius from the downtown area?


  [1]: http://www.bbc.co.uk/london/content/articles/2005/08/15/charingcross_feature.shtml