As we are seeing, where London starts and ends depends on many things, some tangible and others not. The Roman City, The Medieval City, the Georgian, the Victorian, the Edwardian. Whether it is urban or rural. The political geography of the 32 boroughs, or the Post Office's definition, where for example the vast endless metro-land of the north-west is given a Harrow postcode to make it bloody clear it isn't London. Nor is Romford FFS.

And let's face it, if the post office should know where London is. They know where everywhere is.

For me, it's the North Circular. Not sure about Down There but I am working on it slowly. eg: The river marking the border between Tooting and the borough of Croydon is definitely the edge of London.