User:RHaworth/History of London telephone numbers

All figure exchange codes
Letter codes were abandoned in anticipation of STD. Some letter codes were changed to a different numeric code. This was to support seven new sector switching centres (SSCs) to serve exchanges outside the central area so that calls from outside the director area did not all need to go through the middle of London. It was arranged that the first two digits of a London number were all to go to one sector. This meant that the STD equipment in non-director exchanges could treat London numbers like any STD code.

http://www.samhallas.co.uk/repository/journals/POEEJ/POEEJ%20Vol%2067%20Pt%201%20April%201974.pdf