The boundary between his North and South roughly follows the Roman Fosse Way from Lincoln to Exeter.

The 'ditch' (fosse) and road marked the border of the initial phase of the Roman invasion after 43 AD. It was a military road for rapid deployment of forces from the legionary bases to defend border incursions.

https://en.wikipedia.org/wiki/Fosse_Way

Apart from the Fosse Way, almost all other major Roman roads fanned out from London. This structure is still visible in the numbering scheme of roads today:

https://en.wikipedia.org/wiki/Great_Britain_road_numbering_s...

The natural routes for roads also influenced railways, so the boundaries of the original private railway companies (GWR, LNER, LMS, SR), and the regions of the current unified rail system, follow the same structure:

https://www.railwaygazette.com/network-rail-regions-take-ove...