I have summarised some Numbering Changes (past present and future) to the UK numbering scheme
I hope to add more on various aspects of the history of UK telephone numbering
OFTEL - The Office of
Telecommunications - was responsible for regulation of
telecommunications in the UK.
The duties of Oftel have now been inherited by the Office of Communications (Ofcom).
The Phone Information site formerly at Warwick University has been superseded by UKPhoneinfo.com
eDQ - British Telecom's Online UK Directory Enquiries
The uk.telecom newsgroup contains valuable discussion (at times). It is well worth checking their FAQ
The Telecommunications Heritage Group
Light Straw - another valuable site for UK telephony history
There are pictures of Telephone Exchanges of the South Midlands on John Mann's site
Read all about the North American Numbering Plan (NANP)
LincMad - Linc Madison's Guide to Telephone Area Codes and Splits in North America