Hi Steve
The Campaign dates will drop off the navigation map when their dates end.
Your home base will stay as Wilhelmshaven.
You would start your game from the officer without selecting any missions as there will not be any.
You might find this mod useful as it will give you a VIIB type and you will not need to worry about upgrades.
Just enable it next time you are docked.
VIIB U-Boat Replacing VIIA at 31st August 1939 used with OHII v2.5 (See Document Instructions)
You will be free to roam wherever you want and returning to base to take on supplies.
Peter