London borough boundaries
London Borough Boundaries are the key admin boundaries within the greater Greater London area, subdividing it into 32 boroughs.
|Barking and Dagenham|
|Hammersmith and Fulham|
|Kensington and Chelsea|
|Kingston upon Thames|
|City of London||See note on London Bridge below|
|Richmond upon Thames|
|Southwark||See note on London Bridge below|
Note that most online maps showing the position of the borders are copyrighted. Even supposedly open licensed maps on wikipedia are actually derived from copyrighted sources. As of February 2013, all the boundaries are mapped from the most accurate source which we can use at the moment - OS Boundary Line data. Ideally we'll keep it clean by surveying signs in the streets. Keep a look out for those "Welcome to London Borough of X" signs while you're out and about.
According to the OS Boundary Line data, the boundary between Southwark and the City of London is roughly the centre-line of the Thames. However the City of London Corporation controls (through Bridge House Estates) the whole of London Bridge so the boundary has been adjusted in OSM to cover the whole bridge. A similar situation exists at Blackfriars Bridge, although this is reflected in the OS data.
The OSM Inspector tool is excellent for checking that a boundary is complete and that all the tagging of the ways and relation is correct. A relation of London Borough Boundary relations is available for reference.
Wikipedia has a rather comprehensive list of boundary changes within London, which links to the relevant governmental orders which made the change. Although many refer to maps not included, the explanatory notes may assist mapping of both the Greater London and borough boundaries.