Don't forget the other complexities...

There is N, E, SE, SW, W and NW. But not NE (Newcastle) or S (Sheffield). There is a WC and EC, and some codes are later subdivided (e.g. N1 and N1C).

The postal county of London doesn't match the official county of Greater London, so there are parts of London with their own postcode, which also often stretches beyond the boundary (CRoydon, Bromley, TWickenham UxBridge, ENfield, DArtford, etc, etc). This is much loved by people who say they live in Surrey, despite having been part of London since 1974.