I must be a middle-aged Dad. Whilst perusing the excellent Strange Maps blog just now, I found this very interesting Postcode Map of Great Britain.
In the comments is an explanation of how they got numbered in London which was initially based on the alphabetical order of the area names (except W1, N1, E1, etc which were the areas closest to the City). So, for example, SE2 = Abbey Wood; SE3 = Blackheath; SE4 = Brockley; SE5 = Camberwell, etc.
However, once they needed to create some more postcode areas beyond SE20 it all went a bit wonky, which is why our home in SE23 (Honor Oak Park) is squeezed in between SE4 and SE6. Fascinating (but YMMV, obviously...)