A Vision of Britain Through Time, in the GBHGIS, provides maps and statistical information derived from census reports covering the period 1801-2001, searchable by place name or post code.
Digital Ordnance Survey (OS) maps of the UK from the mid 19th century to 1970s are available online at Old Maps.
A Vision of Britain Through Time, and Old Maps, can both be accessed via the Genuki Gazetteer: Enter the place name and click Search; click the pin on the map (NOT the single item on the list top right); pick the third item "Other maps"; pick "OldMaps" (not "Old Maps On Line"); zoom in as necessary; pick a suitable date and scale from the maps along the right hand edge.
The Landmark Information Group sell online 19th Century Ordnance Survey maps linked from an extensive gazetteer. These are at a scale of 1:10,560 (6 inches to the mile) and cover all of Great Britain (England, Scotland and Wales).
Catalogue of British Town Maps - "locates town maps extant in UK public archives and libraries. It provides details of almost 8,000 maps and provides for each the key cartographical and other features and the location of publicly-accessible exemplars. Associated with each catalogue entry is a PDF image outlining the area covered by the map."
Kain, R.J.P. and Oliver, R. British Town Maps: A History. British Library Publishing Division (2015) 240 pp. [ISBN-10: 0712357297]
Genmaps - a very large collection of scanned maps of England, Wales and Scotland.
County maps, showing the parish boundaries, the probate jurisdiction, and the date of commencement of registers, are available from The Institute of Heraldic and Genealogical Studies .
Digimap - a subscription service available to UK Higher Educational Institutions that provides online access to "all available County Series maps at 1:2,500 and 1:10560 scales published between 1843 and 1939; and all available National Grid maps at 1:1,250, 1:2,500 and 1:10560/10,000 scales published from 1945".
FamilySearch's Maps facility can be used to find parishes and see their parish boundaries overlaid on a modern Ordnance Survey map