I think it's important to always keep in mind that the Standard states the following:
"The action must, however, be unrestrained, free and vigorous." It's like a mantra: MUST BE, UNRESTRAINED, FREE AND VIGOROUS.
To me that means, however the dog moves (straight out in front, coming in a little, etc.), it MUST move UNRESTRAINED, FREE AND VIGOROUS.
I feel the Standard is actually silent, unfortunately, on the details of how the legs are to move when the dog is going from Point A to Point B. But, based on the Standard's description of how a Bulldog is to be built and look like, we have a generally accepted guideline for the Breed's locomotion.
The Standard does also indicate that Bulldog Type can be defined, in part, by the "characteristic 'roll'."
P.S. I believe the Bulldog Breed Council in the UK added the following to their Standard: