A Vision of Britain through Time
Includes access to the data from the GB1900 Gazeteer, which is: "The GB1900 project computerised all the place names and other text on the Second Edition County Series six-inch-to-one-mile maps covering the whole of Great Britain, published by the Ordnance Survey between 1888 and 1914 -- 1900 for short. We are making the resulting data available for download in three formats: These are all large zip archives which may take some time to download. They each include documentation and a copy of the relevant Creative Commons licence."
The website can create maps that visualize the population data between the years available (1801-2011) by location or topic, or the data can be downloaded from the website as raw data.