"The system of London postal districts predated the introduction of postcodes throughout the United Kingdom in the 1960s. The first system, of ten sectors identified by letters, was introduced in 1858; the numbered subdivisions date from 1917. The 1917 subdivisions remain important, because they form the first part of the two-part modern postcode (so N1 1AA is an address in the old N1 district), and because they continue to be used by Londoners to refer to their districts."
"The London postal districts rarely coincide with the boundaries of the London boroughs (even the older, smaller metropolitan boroughs). The numbering system also appears arbitrary on the map: for example, NW1 is close to central London, but NW2 is a long way out. This is because (after starting with 1 for the area closest to the centre) they were numbered alphabetically by the name of the main sorting office."
"The BBC soap opera EastEnders is set in the fictional postal district of E20."
"A further complication is that in some of the most central London areas, a further gradation has been necessary to produce enough postcodes, giving unusual codes like EC1A 1AA. While most postcodes are allocated by administrative convenience, a few are deliberately chosen. For example in Westminster:
* SW1A 0AA - House of Commons
* SW1A 0PW - House of Lords, Palace of Westminster
* SW1A 1AA - Buckingham Palace
* SW1A 2AA - 10 Downing Street, Prime Minister and First Lord of the Treasury
* SW1A 2AB - 11 Downing Street, Chancellor of the Exchequer
* SW1A 2HQ - HM Treasury headquarters"
"All Head District Sorting Offices, except London South East, were connected by and had stations on the Post Office Underground Railway."
http://en.wikipedia.org/wiki/Postcode#Postcode_history
http://en.wikipedia.org/wiki/London_postal_district