I think that Bismark was too shrewd to try to take on Britain and France at the same time. Don't forget, in the 1860's Prussia also faced strong challenges from Austria, Denmark, and Poland. And the Prussia which defeated France in 1870/71 was far larger and more powerful than the one which existed in the early 1860's.
Also, the definition of a world war is one which involved ALL the great powers at the time. So a true world war would have to involve Great Britain, France, Russia, China, and the USA. Coincidentally these are still the greatest powers in the world today!