Found this site that has OS 6 inch to 1 mile old map which is overlaid over a map of today.
Just use the map slider to blend between the 2. Very interesting as the old map is quite accurate to todays map and you can see the positions of new and old better than a side by side layout.
It covers all of mainland Britain. Click here
The site has many other resources but a lot of it is behind a paywall.