This may be of as much or as little interest to those far away from it as relative locals. I hope it may be interesting through its total obscurity.

The London postcodes are arranged in alphabetical orders (except for 1's which are the nearest to the centre).
To give a short(er) list, for NW (North West)

NW1) Camden Town (closest to centre) first in name by coincidence
2)Cricklewood
3)Hampstead
4)Hendon
5)Kentish Town
6)Kilburn
7)Mill Hill
8)St Johns Wood
9)The Hyde
10)Willesden Which nicely leads me to 'exceptions'
11)Golders Green
This is because NW11 was partitioned from NW4, plus SE28 which was partitioned form SE2 (Abbey Wood) and SW20 (Raynes Park) which may have been added rather than split from SW19 (Wimbledon).

Most codes contain a few different towns, though since the codes with numbers began maybe 100 years ago, few records exist showing the internal boundaries, therefore allowing real estate agents to move the boundaries at will. For example NW10, being large and well-populated, contains Willesden plus Neasden, Park Royal, Harlesden, College Park, Kensal Rise and Kensal Green.
Sometimes (as postcodes are solely to get your letters to the right place rather than name your area) places straddle two or more postcodes, such as Neasden and Willesden Green. Two London codes, E4, Chingford and NW7, Mill Hill, even stretch beyond London into Essex and Hertfordshire. A good part of E4 is outside London completely, but NW7 is all inside it except for a small corner.

Prior to this, the smaller County of London just used the letters- N, NW, E, SE, SW, W plus the centrals EC and WC.

My own, NW9, is quite unusual, in that though it is split between Kingsbury (my bit), West Hendon (divided by borough boundary) Colindale (divided by main A5 road) and The Hyde, which is the name of the A5 right in the middle of the NW9 area and a little area around it, The Hyde (its correct title) has more or less fallen out of use except for official letters that have taken it from the Post Office list. If you told someone you lived in The Hyde, no one would know where it was, except for a few professional drivers maybe. All the other official names for London codes except The Hyde are in common use.

Someone was telling me that the post code (not necessarily London), for example NG27 6OR
could tell me exactly where the postie had to go - right down to the last street. Is this true?

If it does, please don't give the address away, as it is likely to upset my friend greatly.

Edited to add: I have changed the post code that I originally had to protect the innocent, to something that probably makes no sense at all and hopefully won't upset the thread too much.

Yes it is true, for instance I know immediately that your friend lives in the same area as my sister!

Now take my own post code, the last two letters are EN, two roads away the code is EL. For Jersey (which uses the UK system of post codes) the JE stands for Jersey, 1, 2, 3 or 4 tells you whether it is in town, country or a post box number. Then we have another number, that tells you which area of the Island, the last two letters give the road. When sending parcels I give the return address as the house number and my post code, I do the same on suitcase labels.

Thanks for the post on London Postal Districts, Sat. Actually, it's just a shade messier than meets the eye. London SW is (or was) in fact divided into two completely separate sections: SW1-10 (which includes some areas south of the Thames, such as SW2 Brixton, SW4 Clapham, etc.) and then SW11-20 (all south of the river). The District (or Head) Office (D. O.) for the former is SW1 (near Victoria), while that for the latter is the Battersea District Office (near Clapham Junction). For SW11-20 the numbering starts all over again in alphabetical order: SW12 Balham; SW13 Barnes ... till one gets to SW19 Wimbledon - followed by the SW20 Raynes Park anomaly.

In the case of London W, W1 counts as a fully fledged Distict in its own right, with the second London W District Office W2 near Paddington for W2-14.

The District (or Head) Offices and the offices in EC and WC are all unnamed, except for the Battersea D. O (SW11) the Paddington D. O (W2).

The original division of London into postal districts dates from 1856 and is much older than the creation of the old County of London (1889), so it's not at all suprising that the London Postal Districts and the boundaries of the old County of London didn't coincide. Moreover, the divisions in 1856 were made by the GPO (Royal Mail) for its own convenience and didn't correspond to the local government boundaries of the day. For example, even in those days it was not unknown for one side of a street to be in one parish and the other side in another. Obviously, to have had one side of a street in, say, EC and the other in N would have been most unhelpful.

The original division was simply into London EC, WC, W, NW, N, NE (yes, but merged with E in 1860(?) ), SE and SW.
These divisions went hand in hand with the establishment of what in the 1850s were called 'suburban' sorting offices. Before that, nearly all the mail in the London area was sorted centrally at the main GPO in King Edward Street/St. Martin le Grand, but by the early 1850s this was leading to serious delays ... The original District Offices were, on the whole, located close to major railway termini: for example, the NW D. O. was in Eversholt Street, near Euston.

Unfortunately, the first maps of the divisions published in 1856 weren't well printed and were particulalry unclear - just in case people didn't guess - at the very points where they needed to be clearest, that is the boundaries. Oh dear ... oh dear.

The GPO made some adjustments and in 1860 virtually "finalized" the boundaries where they are today. (As far as I know, it was then that London NE was merged with E and that Edgware was taken out of London NW). Quite early on, the GPO woke up to the fact that if they wanted individuals and business to put the postal districts in their letter-heads it was essential to stop fiddling around with the district boundaries. As far as I'm aware, the only major change was in 1899(?) when some streets in NW were transferred to WC.

I *think* the further division into numbered sub-districts was devised in 1913 but didn't come into effect till after WWI.
I'm less certain about this, and I'd welcome corrections on this ...

Yes, I love geography trivia, talking about split roads, where I used to live had the parliamentary boundary down the middle. People opposite could vote Mrs Thatcher back to become prime minister, being in Finchley. We had a very nice MP Peter Thomas, who came to my school, in Hendon South. You've reminded me of some more old road facts I'd forgotten about, and I'll post them under 'Typical English roads' now, and add any more I remember later on.

Exit, yes, you're absolutely right! The British postcodes each cover a very small area in urban parts of the country, anyway. A postcode may cover only part of a street and, as originally conceived, every purpose built block of flats had its very own postcode, so those things can be very revealing indeed. For example, the block of flats where I live in Norwich has its very own postcode, despite the fact that it consists of only 12 flats! The neighbouring block of flats, which also consists of 12 flats, also has its own, different postcode.

Market researchers, some people trying to sell things by phone, and even political parties sometimes think they know all kinds of things about one, simply on the basis of one's postcode.

Good heavens! You Brits have a complicated mail delivery system. Do the post office people have to memorize all these addresses as do the London cabbies?
I thought that the U.S. ZIP codes were unwieldly, but they are comparative child's play to yours.

It's a very ambitious system. When the mail is sorted the postcode is typed on the envelopes, etc., just as in other countries, so no heroic feats of memory are necessary.

P. S. I've a hunch the system of postcodes in the Netherlands is rather similar.

I've noticed that the London SE postcodes do something a bit odd at SE19 - Norwood. With SE20 - Anerley, SE21 - Dulwich, the alphabetical numbering in SE starts all over again. This suggests that the original plan envisaged creating a District Office in Norwood, rather along the lines of the Battersea D. O. However, as far as I know, the GPO never acted on this. It could just have something to do with the lengthy gap between the plan for the numbered areas and the time when they came into effect, but obviously this is a guess of mine and nothing more. Moreover, at every stage the GPO had the tricky task of trying to predict future patterns of development in the London area.

I'm amazed you managed to dig all this out- I heard the formula and matched the names to the numbers myself, then learnt the additions. But, being a foreign land, I hadn't worked out the South London codes so if I had I would certainly have got stuck!
On the complex codes point, as far as I know the Canadian postcodes are at least as complicated as ours though I don't know what all the letters and numbers represent.

Well, South London has traditionally been the "poor relation", so to speak. I'll never forget the arrogance with which I heard someone declaim at a party, "It's just not done to live south of the River!" I was tempted to ask about parts of Greenwich, Clapham, Wimbledon and Richmond, but then decided there was no point ...

More seriously, I suppose that when the basic postal divisions were made by the GPO in 1856-60 many of the guesses about the future development of London south of the Thames were simply mistaken. The vast South London rail commuter network didn't exist at the time.