These links will help you investigate the maps availble in The National Archives: Looking for a map Maps: further research or check their other research guides
The Ordnance Survey produce a wide range of useful maps. Also see The National Archives guide to Ordnance Survey Records .
Reproductions of many early large-scale Ordnance Survey maps for many areas are available from Alan Godfrey Maps .
In a somewhat random order.
The National Library of Scotland has put numerous maps online inluding Ordnance Survey maps for England and Wales from c.1900 which can be overlaid on current maps.
Gardiner's A School Atlas of English History (edited by Samuel Rawson Gardiner, M.A. LL.D. and printed in London by Longmans, Green, and Co. in 1892).
Note that from late 2021, these maps are no longer free: The Landmark Information Group have provided 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). (Maps vary between 1840 and 1890.)
Lewis' (c.1840) maps of England & Wales have been put online by MapCo.
Maps of England and Wales in