MapIt is "a service that maps geographical points to administrative areas. It is useful for anyone who has the co-ordinates of a point on Earth, and who needs to find out what country, region, city, constituency, or state it lies within. It’s also great for looking up the shapes of all those boundaries."
There is an edition of
MapIt based on source data from OpenStreetMap and another (
MapIt UK) based on the open Ordnance Survey data. Generally the service has a cost dependent on usage, but charitable use up to a limit is free.