Ordnance Survey is the British national mapping service. It gets a mention in PP.
Ordnance Survey maps can be used on this Wiki if they are old enough (50 years old or more). Upload an image of the map and make sure you add the following statement to your description:
This work is in the public domain because it is an Ordnance Survey map over 50 years old. Ordnance Survey maps are covered by crown copyright which in this case expires 50 years after publication. Ordnance Survey does however ask that they be credited and that the date of publication be given. Any ancillary rights gained through the creation of the electronic version are granted as freely usable under any circumstances.