Many accurate 1921 census transcriptions have already been added to the pub history site, including London, and parts of Middlesex. If you can contribute to the site, send me some details to Kevan, along with some description of the specific property.
My favourite mapping site is at: Mapping Layers of London
I am extremely grateful to David Hale for allowing me to link to his maps at the MAPCO site , a fantastic resource.
Here is the Scottish nls site (national library of Scotland) - There are over 86,000 maps as high-resolution, colour, zoomable images. The maps date between 1560 and 1961 and relate primarily to Scotland, but also maps of England and Great Britain, Ireland, and Belgium.
I love this site which allows a comparison between the mappings of 1746, 1869 and today - brilliant. At locatinglondon,org
There are a considerable number of maps on the mapco site which are absolutely brilliant, e.g. in 1857
The assorted Maps of Tower hamlet is an absolutely brilliant resource I have been keeping to myself - it allows simultaneous views of different peiods in time - i.e. 1745, 1766, 1801, 1802, 1822, 1831, 1844, 1853, 1882, 1952 & 2000 for bethnal Green & Stepney.
And more Maps of Tower Hamlet in 1745, 1801, 1831, 1853, 1861, 1875, 1882, 1952 & 2000 for Bow, Bromley & Poplar.
Other miscellaneous