Loz said It's not official, as there is no official place called Middlesex any more (unlike Kent, which has a county council). And some part of the old Middlesex, like Ealing, do have West London postal codes (e.g. W13, W9, W4) and much of the rest NW postal codes. The rest of it has Uxbridge (UB], Harrow (HA), Watford (WD) and Enfield (EN) codes. It isnt unofficial either! Those 4 places are all entitled to use Middlesex, dating from the day they were incorporated in the (then) new county of Greater London. It's a historical county and all over the country people - if they so wish - can include the old county's name in their addresses. And if they don't so wish, they can just put the post code.