Apparently this is a coalhole Coal holes: These are covers from the period when the houses were heated with coal, into which the coal bags were poured, so that the coals did not have to enter the houses. These lids were used in the 19th and early 20th centuries.
The same cap but without drainage holes were photographed on 2 Fitzhardinge St, Marylebone, London W1H 6EE, UK Click for a larger image