I was very disappointed when both Oakley publishing and BT stopped printing their code decoders in unison and besides the Oftel site which is hard to navigate and not always up to date, when I see new numbers (they stand out like coins in the street to me) I can't always find the exact meaning of them. I would like to complete my local 020 list with the new 020-3 exchanges without waiting to see a number and marry it to its location each time. I go back to the days when phone books listed every exchange location for London and like to maintain it. If I read an ad in the paper I'd still like to know if it's in Barnet or Bexley and despite muddying the waters greatly I can still do this for non-BT exchanges as well albeit within a much larger area.

Is there any other publication to replace what has gone or will we all now have to rely on the internet or asking insiders? I am still waiting to hear where 020 8457 is for as it came out since the last book was published (although most users are in the 8455 area it may well include others as many do nowadays). I don't like gaps in my data and am proud to be the anorak that I am. And why should we have to be content with partial data anyway? You wouldn't want it from a professional?

this might also be interesting or useful

http://www.magsys.co.uk/telecom/codelook.asp

which shows for example that the 020 8457 ~ you were wondering about are BT numbers in Hendon (~ 1-4, 6, 9) and Golders Green ( ~ 5, 7, 8)

I think a lot of the 020 3~ numbers are allocated to other providers which don't necessarily imply a particular geographic location - or they aren't defined on there; how numbers or lines get ported, I don't know

I use an old (2001) edition of BT's Phone Book Companion and note that BT are still selling it for £4:  http://www.shop.bt.com/productview.aspx?quicklinx=4KBG.

This publication is probably not as detailed as you would like, and I am not sure how up to date the current edition is, but at least it is a book.