I could (previously) say I lived in D9, 10, 11, 12, 15, 21 etc. all of these were clear and concise.... 'Ooh D10 look at him, (bloody snob) !!

Right now I look at my 6-digit postcode. What does it men? Does it define an area... I suppose so. Would anyone know what the numbers mean? No idea...
Hence why the old 'D[x]' prevails, it says so much more, together with the sub-text...