I wrote this last year after a visit to the British Postal Museum Archive about how postcodes originated....
A few weeks ago I attended a talk by Helen Kearney at the archive Search Room about the evolution of the postal map and it's importance in history. The original postal map was drawn up by cartographer Edward Stanford in 1856 at the request of Rowland Hill and contained 10 postal districts which all bar two are still used today for the first part of your postcode. Originally we had Northern, North Eastern, Eastern, South Eastern, Southern, South West, Western and North Western Districts. The two that were lost were the North Eastern District and the Southern District in 1865 and 1866 respectively.
This meant that there could be daily deliveries between each District Office rather than mail coming into the centre of London and then having to be delivered out again once sorted!
The map has a radius of almost 12 miles measured from the original Central Post Office of St Martin's Le Grand very close to St Paul's Cathedral and has only slightly changed to this day. The numbering of postal areas was a few more years in coming despite repeated attempts since 1911. It was WW1 and the inexperienced temporary sorters that eventually meant that the District plus number system was introduced.
We learnt from Helen that the area containing the District office was always linked to number 1…N1, SW1 etc and then areas within a district were put in alphabetical order and numbered consecutively after that. This means that two adjacent areas in a postal district could be numbered N1 and N10!
A reminder of these now disappeared postal districts can be found on a few unchanged streets signs, such as Stamford Grove West in Stamford Hill which still has NE displayed on the sign.
It was an interesting talk and a previous version can be listened to on the BPMA website.