I'll add some more bits and pieces. Every postcode stands for something (North, North West etc within London) or somewhere outside except IG for West Essex, and SM, though I was told it was for Sutton and Merton.
All numbers are usually arranged in alphabetical order following 1 for central. So Whitefield, being near the end is Manchester M 45. Some centres became so big they were split in two, where the extra code is designated 0, as in CR0 (central Croydon) or HA0, part of Wembley (this makes little sense as it's not in the centre of Harrow at all).
In London three postal areas grew too big and were split in two, NW4/NW11 SW19/20 and SE2/28, so the final number is not in alphabetical order but an addition. Finally one of the two S codes was divided in two aplhabetically, I think it's SW.