The ultimate origin is obviously geographic and topographical, as first laid out by the Romans. The region 4-5 boundary along the A5 is just Watling Street. It

The ultimate origin is obviously geographic and topographical, as first laid out by the Romans. The region 4-5 boundary along the A5 is just Watling Street. It originates in London today as Edgware Road, which preserves its distinctive Roman straightness (it even has the faux-Roman Marble Arch at the start in Hyde Park): https://en.wikipedia.org/wiki/Watling_Street https://en.wikipedia.org/wiki/Watling_Street https://en.wikipedia.org/wiki/Edgware_Road https://en.wikipedia.org/wiki/Edgware_Road The obvious exception is the boundary of the phased Roman invasion, which cuts diagonally from south-west to north-east: https://en.wikipedia.org/wiki/Fosse_Way https://en.wikipedia.org/wiki/Fosse_Way The Fosse Way was originally a military road to support the eponymous barrier 'ditch' and rampart. Now it is mostly (near) the course of the A38 which does not follow the radial region 3 and does not fork from the A3 to the SW of London. The regions are also approximately the division of Anglo-Saxon kingdoms: Sussex ( South Saxons, region 2), Wessex ( West Saxons, region 3), Essex ( East Saxons, region 1) and Mercia (er... midlands, for all-the-other-Saxons, regions 3~4): https://en.wikipedia.org/wiki/Heptarchy https://en.wikipedia.org/wiki/Heptarchy Watling Street later became the border between Viking and Anglo-Saxon sectors of the island (hence the radical divergence in accents and place names on opposite sides of the road): https://en.wikipedia.org/wiki/Danelaw https://en.wikipedia.org/wiki/Danelaw Also note that because it is essentially topographical, it was also followed by the private railways, giving LNER (region 1), SR (region 2), GWR (region 3) and LMS (aforementioned Mercia and west coast line, ~ regions 4,5,6), hence the originating stations in London: https://en.wikipedia.org/wiki/London_and_North_Eastern_Railway https://en.wikipedia.org/wiki/London_and_North_Eastern_Railw... https://en.wikipedia.org/wiki/Southern_Railway_(UK) https://en.wikipedia.org/wiki/Southern_Railway_(UK) https://en.wikipedia.org/wiki/Great_Western_Railway https://en.wikipedia.org/wiki/Great_Western_Railway https://en.wikipedia.org/wiki/Southern_Railway_(UK) https://en.wikipedia.org/wiki/Southern_Railway_(UK) P.S. Direct links to Wikipedia should be banned from HN submissions.