"Taking back that land shouldn't require much fighting."
Great Britain becomes more mountainous and forested as one moves east, with Cornwall being a mountainous region. However, to the west, it's mostly plains. In the early days of the empire, paved roads were built in the British Province, with more than ten tribal-level administrative districts and over thirty towns established to facilitate urban management and military deployment.
A tribe is roughly the size of a small kingdom, approximately equivalent to the territory of a county.
After capturing Devon, moving towards London means facing large-scale legion warfare, and the Saxon Barbarian Race will find it easier to mobilize their troops.
However, the plains are more challenging to defend once captured.