OS Six Inch Road Maps
The map room on the second floor of the National Archives contains a comprehensive set of six inch / mile Ordnance Survey maps dated from around 1935 - 1940. They have road numbers, including some quite obscure B roads that can't obviously be found from elsewhere. The maps are organised with main sections for England, Wales and Scotland, then by (traditional) county in alphabetical order.
You don't need to order any of these maps (though you do need a reader's ticket to get into the map room), just pick one up and open it up on one of the flat tables.
The maps cannot be copied without prior special arrangement.