The "area code" is also referred to as an "STD (code)" (subscriber trunk dialling) or a "dialling code" in the UK.
The code allocated to the largest population is (020) for London.
That is, almost all (01xxx) area codes now have only six digit local numbers, e.g.
(a small selection) Six of the four-digit area codes are known as "mixed" areas as they share those four digits with the twelve five-digit area codes.
The Big Number Change removed many, especially in Northern Ireland, and by July 2005 there were only 329 ranges in 42 codes.
By April 2010 this had reduced to 324 ranges in 40 codes, with the same number still in November 2012. This is the geographic number format for the first round of five large cities moved to all figure dialling in the 1960s, and subsequently also used in the metropolitan county of Tyne and Wear, eastern County Durham and south-eastern Northumberland from the 1980s onwards.
The 0845 range can have NSN length as 10 or 7 digits. There are no telephone numbers in the UK with an NSN length of 8 digits. It has a four-digit area code (after the initial zero) and a six digit subscriber number, and is known as 4 6 format.