A British area code is two to five digits after the trunk 0 — 020 London, 0131 Edinburgh, 01223 Cambridge, 015394 Hawkshead — and there are some six hundred of them. The plan of 1958 spelt the town on the dial: 2 stood for A, B and C, 3 for D, E and F, and so on, which is why Cambridge is 01223 (CA) and Bath 01225 (BA), and why neighbouring codes are scattered across the country. At full view the map shows the regions, each with its principal codes; zoom in and the codes themselves appear, each on the parishes and zones whose places dial it, coloured by its letter group.
How to read the map. At full view each patch is a region of England, a nation, or a Crown Dependency, and the numeral on it is the region's principal codes — the short city codes it holds, or Cambridge and Norwich for the East of England, which has none. Zoom in twice and the codes take over: each patch is then a code and the numeral on it is what you dial there, with the place the code is named for beneath; zoom in further for the smaller ones. The colour of a code is the digit after the 01, which in the plan of 1958 stood for three letters on the dial — 2 for A, B and C, 3 for D, E and F, 4 G H I, 5 J K L, 6 M N O, 7 P R S, 8 T U V, 9 W X Y — so that a code spelt the first two letters of its town: 01223 CA(mbridge), 01225 BA(th), 01387 DU(mfries). The five wide 02x areas of 2000 (020 London, 023 Southampton and Portsmouth, 024 Coventry, 028 all of Northern Ireland, 029 Cardiff) and the six 011x cities of 1995 (Leeds, Sheffield, Nottingham, Leicester, Bristol, Reading) spell nothing and share one colour. Twelve codes have five digits — 013873 Langholm, 015242 Hornby, 015394 Hawkshead, 015395 Grange-over-Sands, 015396 Sedbergh, 016973 Wigton, 016974 Raughton Head, 016977 Brampton, 017683 Appleby, 017684 Pooley Bridge, 017687 Keswick, 019467 Gosforth — each a part of the four-digit area it begins with, and drawn inside it. The Isle of Man (01624), Jersey (01534) and Guernsey (01481) are not in the United Kingdom but are in its numbering plan, so they are on the map.
Where the lines come from. Ofcom publishes each code with the town it is named for and no boundary; an area code follows the old telephone exchanges, not any administrative line. The map is drawn on the finest public units that cover the country — the civil parishes of England and Wales, with the unparished towns as areas of their own, and Scotland's intermediate zones; Northern Ireland is all one code — and each takes the code that most of the places Wikidata puts inside it dial, from the dialling code recorded on some ten thousand towns and villages. An area with no such place takes its nearest neighbour's code, which the build counts. So a patch edge is a parish edge, and a code whose true limit runs between two villages ends here at the parish line between them. Codes no area dials by majority are in the list and inside a neighbour's patch, not drawn as patches of their own.
Sources. Codes: Wikipedia, List of dialling codes in the United Kingdom — the geographic codes with the place each is named for, after Ofcom's National Telephone Numbering Plan. Code per area: Wikidata, the local dialling code (P473) and coordinates (P625) of every place in the United Kingdom that records one; located in its parish or zone by point-in-polygon. Geometry: ONS Open Geography Portal, Parishes and Non Civil Parished Areas (December 2024) EW BSC and Countries (December 2024) UK BSC, Open Government Licence; Scottish Government, Intermediate Zone Boundaries 2011, Open Government Licence; geoBoundaries (Isle of Man, Guernsey) and OpenStreetMap via Nominatim (Jersey), ODbL.