why is london zip code complicated

The complexity of London’s postal codes stems from their long, piecemeal historical development and a specific attempt to avoid social snobbery in the numbering system.

1. Historical Evolution (and Abolition)
• Original Compass Divisions (1857): The system began in 1857 by dividing the growing city into two Central areas and eight compass points: EC (East Central), WC (West Central), N, NE, E, SE, S, SW, W, and NW. 
• The Missing Codes (NE and S): The complexity increased early on when the NE (North East) and S (South) districts were abolished and merged into their adjacent districts (E, SE, and SW). This is why you won’t find London postcodes starting with NE or S—those letters were later allocated to other cities (NE for Newcastle and S for Sheffield) in the national system. 
• Subdivisions Added (WWI): During World War I (1917), numbers were added to the postal districts (e.g., N, E, SW) to create sub-districts (e.g., N1, E2, SW6) to help temporary staff sort mail more efficiently. 
2. Lack of Geographical Logic in Numbering
The most confusing aspect is how the numbers are assigned within the postal districts (N, E, SE, SW, W, NW):
• **Central Districts are “1”: The number 1 in each compass area (e.g., N1, SE1, W1) is generally reserved for the area closest to the centre of the city, often where the main sorting office was or is located. 
• Alphabetical Ordering: Beyond the “1” district, the remaining numbers were intentionally assigned in alphabetical order based on the name of the locality or delivery office at the time. This was done to be perceived as a fairer system and to prevent high numbers from being stigmatized as being the furthest and least desirable parts of the city. 
• Result: This means an area with a low number like SE2 (Abbey Wood, which is far out) can be geographically distant from SE11 (a central area near Westminster) because “Abbey Wood” comes early in the alphabet. 
3. Sub-Districts and Outliers
• Further Sub-Divisions: In very dense areas, the postcode district (the first part of the code) is sometimes further split with a letter, creating sub-districts (e.g., W1A, EC1A). 
• Outliers: More recent developments, like the Olympic Park (E20), have been assigned numbers that break the original alphabetical sequence due to new large-scale housing or commercial construction. 
• Outer London Postcodes: Many parts of Outer London use postcodes for their original non-London “Post Towns” (e.g., a suburb in the London Borough of Croydon might have a CR postcode for Croydon, not an SE or SW London code). 
In short, the postcode system is a mix of old compass divisions, wartime efficiency measures, a conscious effort to avoid social snobbery, and a few later ad-hoc additions, resulting in a map that often defies simple modern geographical logic.