Sorry to disappoint you but I would be surprised if there is actually 6mm of cable in a 12mm cable lock. I bought a used bike once with said lock wrapped around the seat tube and the keys long lost by the owner. I took it home and it took me seconds to cut it with a disc on an electric drill. The (transparent) plastic sheaf of the lock acts like a magnifying lens, making the weeny steel cable inside looks much bigger than it actually is until you can see it in cross section!
On the other hand, if you were given a 12mm steel cable, I wonder how flexible/useable a cable it would be.