Factual error: Assistant Commissioner Dove's accent is working-class London. Although he is a fictional character, it is inconceivable that an assistant commissioner in the 1890s would sound like this (or be as young as he is). All officers above the rank of superintendent in those days were men who were transferred directly into senior rank, having previously had long and distinguished careers in senior ranks in the army, colonial police, civil service or law. The first man to be promoted to senior rank from the lower ranks was James Olive, who was promoted to assistant commissioner in 1920 at the age of 64 and after 48 years service.
Factual error: It is implied throughout the show (and explicitly stated a couple of times) that Reid is in command of H Division. From the earliest days of the Metropolitan Police, a division was actually commanded by a uniformed superintendent, not a detective inspector.