Group From 29/9/1829
In 1830 local divisions, one for each borough and each with its own station, were established. These were known by letters of the alphabet. e.g. M division was for Southwark.
Wikipedia states the Met is "the first modern and professional police force in the world" though the establishment of the Marine Police preceded the land-based force by over 30 years, and Commissioners of Police for Scotland were appointed in 1714. It's all in the definition.