So 10 characters max. Actually 11 because I think you could have 123a

You should never have to specify more than 1 number. If you have to say flat 1, number 12 the post office needs to assign you a new number (e.g. 12a).

Still beats the hell out of MARAMUT-GACIRA, especially as post codes give a lot of information (I know instantly whether a postcode is in or outside London, and if inside roughly where)

Instantly, that's impressive? I can only manage that if you define London as "The area covered by the London compass-direction based postal districts" If you choose either "governed by a London Borough Council", or "Within the M25" or "Within the City of London", I'd have to look it up.

The postcode district states which sorting office delivers to the address, leading to upmarket Windsor having a Slough postcode SL5

EC1A - inside The City; EC1M - outside The City.

EN1 - in a London Borough; EN10 - In a Hertfordshire borough.

KT9 - Inside M25 and in a London Borough; KT11 Inside M25 and in a Surrey borough.

Maybe not for everything, and i'm sure there are loads of technicalities on the edge of london.

But for your examples:

EC1A and EC1M are both in or around the city

EN1 is enfield I think? So far north, EN10 I imagine is just further north

KT9 I think is Kingston. Both KT9 and KT11 are probably south west near the edge of london

> I can only manage that if you define London as "The area covered by the London compass-direction based postal districts"

That's actually basically what I meant. Compass postcode = london, non-compass is effectively outside london (or on the very edge)

Either way, the point that post codes carry a lot more information than codes generated from GPS coordinates still stands

OK OK, I lose to pedantry :-)