I believe you -- in the large village where I grew up there was some daft division between having a 5-digit area code for a nearby town rather than the (nearer) large city's 4-digit code, and (along roughly the same lines) having <Town, Countyshire> as your official address rather than <Large city>.

But it seems odd for London, when there are plenty of posh areas in outer London, and run-down areas of inner London. Maybe it was more for businesses to appear 'central'?