Steel mills, not mines, of which Britain is famous for. Sheffield, anyone?
There was indeed an influx of Afro-Caribbean people into the UK prior to and during the First World War. The allure of jobs in heavy industry (steel mills and shipbuilding) and dock works spurred on by the arms race drew many people, of which the Afro-Caribbeans were amongst the larger groups. If you were listening, the narrator did allude to this.
So no, not Africans in suits, but Afro-Caribbeans immigrating to join the work force.