Ordnance Survey

Ordnance Survey has one of the largest collections of historical mapping in the United Kingdom. As a result of a joint venture between Ordnance Survey and Landmark® Information Group, this extensive archive has been scanned to create Historical Map Data. National coverage is available, dating back to the mid-19th century and derived from 1:10 560, 1:10 000 mapping and 1:2500 scale County Series, post-War National Grid, and superseded mapping that includes 1:1250 scale and 1:500 scale Town maps.

http://www.ordnancesurvey.co.uk/oswebsite/products/historical-map-data/index.html

See also Old Maps

http://www.old-maps.co.uk/index.html