Make teleportation your friend.
Before you do anything, make sure you are able to transfer to a Mediterranean port by checking the telephone in your home base to see what's available - you don't want to risk the Straights if it isn't available.
If it is available, transit the Straights (
) and sail to around 100km/50 mi of the port of your choosing -
no closer, then hit the [ESC] key and choose "End Patrol".
If the option given is "Return to base", choose it, and you'll be
teleported to your own home base. You can then use the telephone to transfer back to the Med., and be teleported -yet again- to exactly the same office... but in the Med at your new home base.
If the option given is "dock at XXXXXXX", then you are too close to the port. If you dock, transfers will not be available to you, so sail further away until you get the "Return to base" option.
That's about the least un-immersive way I can think of managing it.