The National Library of Scotland (a fabulous source for on-line mapping) has now added a new batch of highly detailed Ordnance Survey 1:500 “Town Maps” to their website. Several Kent sheets have also been added. These maps date to between c.1850 and c.1880 and are an amazing resource for anyone looking at sites in urban areas. Take a look...
They’re available as a seamless layer overlain onto modern mapping:
https://maps.nls.uk/geo/
Or as individual high resolution scans: