Dartford is defenetly kent. It's to far out of london to be classed as london. Besides, dartford comes under kent fire brigade, and will hopefuly never be part of london. The m25 passes kent which has no part of it in london. Erith next to Dartford is part of london, however it is still classed as kent. So were do you live again?
Also you named 3 counties, essex, kent and surrey. For some reason you put dartford in the middle, is dartford it's own county?