A natural strait through Panama

What changes if the Isthmus of Panama never fully forms and leaves a passage resembling the Dardanelles?

An earlier passage could let Britain establish itself there before the United States.

The counterfactual replaces a later engineered crossing with an earlier natural strait. British positions in the Caribbean could then support access before the United States established its own position. The answer therefore shifts attention from canal construction to who first controls the route.