From the Wiki:
A coal hole is a hatch in the pavement (sidewalk, in US usage) above an underground coal bunker. They are sometimes found outside houses that existed during the period when coal was widely used for domestic heating from the early 19th century to the middle 20th century. In Britain they became largely obsolete within the major cities of the UK when the Clean Air Act forced a move towards oil and gas for home heating.
The coal hole allowed the easy delivery of coal, generally in sacks and often from horse drawn carts, to the house’s coal bunker. The location of the coal hole on the street minimised the distance the sacks needed to be carried and meant that dusty sacks and delivery men did not need to enter the house.
I’ve lived in London for twelve years and never really noticed these small metal plates but now that I know what they are I see them everywhere.