From the Wikipedia page 
Harrow on the Hill was an urban district in Middlesex from 1894 to 1934.
It was created under the Local Government Act 1894. This Act split the historic parish of Harrow on the Hill several ways, with parts becoming the new parishes of Harrow Weald, Wealdstone and Wembley. The remaining part of the parish formed Harrow on the Hill Urban District.
It took in 101 acres (0.41 km2) from the disbanded parish of Northolt in 1928.