When I was at school, I lived a long way up north. My geography of the much of the rest of the U.K. was limited to the AA road atlas my parents had in the car, which I used to look at compulsively during long journeys. I was fascinated by the schematic diagrams showing the layout of road junctions on each of the motorways. The motorways were represented on the diagrams themselves by dead straight lines – with one exception: the M25. This motorway was shown as a square, apparently enclosing all of London. So, for many years, I assumed that the London boundary was the M25 itself. I was a little disappointed when I moved down to the city and discovered this was not the case. Several large areas – Epsom, Loughton, Watford – are comfortably inside the M25 ring but not within the administrative boundary of Greater London. Similarly, the boundary pushes out beyond the M25 in a few, generally rural, places.

It turns out there are a lot of official and unofficial ways to define London’s extent.

• Greater London – the administrative extent, made up of the 32 London boroughs and the City of London at its centre. It is the area that is administered by the boroughs and also forms the area of the six concentric Transport for London travelcard zones – although there are some “special” additional zones which go beyond the boundary. Greater London is shown above.
• Greater London Urban Area – the Office of National Statistics defines this as the conurbation area of London (i.e. the continuous urban environment) which is roughly equivalent to Greater London but excludes the large rural areas within the latter boundary, such as Biggin Hill, and includes some towns which “spill over” the Greater London boundary, such as Staines and Dartford.
• London Travel to Work Area (TTWA) – Travel to Work Areas are contiguous regions containing mainly people that both live and work there. London is such a region, its TTWA extends slightly beyond Greater London to include places with sufficiently good transport links that, as far as employment is concerned, are “local”.
• The extent of the “020″ telephone number prefix – the dialling code for “London”.
  The London postal district – the extent of the SW, W, NW, N, E and SE postcodes. These miss out a surprisingly large part of the London urban area, except in the north, where they even extend beyond the Greater London boundary.
• The County of London – this approximately represented inner London and ceased to exist in 1965 with the creation of Greater London. However many older people continue to refer to the counties that were lost or redrawn to accommodate Greater London, such as Middlesex, which is now subsumed by the northern part of Greater London.
• The City of London – this still exists but only covers the Square Mile – the financial and historic centre of London. It is surrounded by the 32 London boroughs. One of the other boroughs – Westminster – is also a city. Hence the electoral constituency which currently covers both being called “Cities of London and Westminster”.

Personally, I still prefer the M25 as the boundary. If I’m heading on a long cycle ride from the centre of London to (say) Brighton, then its when I pass underneath the M25 – a very tangible, physical feature – that I feel I have finally left the city. None of the other borders described above are represented on the ground, other than by road signs. But you can’t miss a huge 6+ lane orbital highway.

The bottom set of pictures are, clockwise from the top right: The London postal district in red, the London Travel To Work Area in dark blue, the former County of London in green, the City of London in bright red, aerial imagery of London’s built up areas, and the London 020 dialling code area in red. Apart from the top picture, which is from OpenStreetMap, all pictures are sourced from Wikipedia. All the picture here are are subject to Creative Commons copyrights of their respective authors. The middle picture shows Greater London, with the boroughs (and the City of London) numbered.