I've often wondered this - what was the boundary between traditional Surrey and traditional Kent. I read an entry in the DNB the other day that referred to the subject dying in the 1960s at "Longton Avenue, Sydenham, Kent" (quite far west), and Sydenham CC used to play in the Kent league before merging with Beckenham, so that's a big clue.

Further north I understand the line was the old boundary between the parishes of Camberwell and Lewisham, but I've never seen it on a map, though I'm told the border between the boroughs of Southwark and Lewisham does not correspond. Anyone got any better information?