Because iron ores were widespread and iron was relatively cheap, the location of iron mines was often determined by the availability of wood, which Britain had in abundance, to make charcoal smelting fuel. Great amounts of iron were needed for the Roman war machine, and Britain was the perfect place to fill that need.
Iron ores' accessibility and low cost meant that iron mines were typically located where wood was readily available, a resource Britain had in abundance, to use as fuel for charcoal smelting. The substantial demand for iron by the Roman war machine necessitated a suitable location, with Britain being an ideal choice.