It's hard to test bite forces of specific breeds since individual dogs can vary so much themselves, and actual testing for something like that is hard to begin with. I also disagree about the Rottie having the strongest bite, I've never seen anything or heard that to be true.
For the bite of an English Bully though, I can tell you it is tremendous for their size. They were bred to hold onto a bull's nose and not let go for anything, while they have since been bred into more compact dogs for pets, alot of the same structure of the head has remained the same. I work with dogs on a daily basis (humane society) and through the years have been bitten by some of the more unstable or untrained dogs that have ended up with us (all breeds). While getting bit sucks anyway, the one time we ended up with a situationally agressive bulldog, I seriously thought when he got my arm instead of the other dog he was going for, that he could very easily rip at least half of my forearm off with that one bite. He fortunately wasn't aggressive to people and released my arm as soon as he realized he had gotten someone other than the dog he was aiming at in the kennel, leaving just a deep nasty puncture wound that got stitched up. It was one of a very, very few dogs I was impressed and scared of the strength of the jaws. (including pits, rotties, and other commonly used fighting breeds we get from busted up fighting rings.)