Boulogne was not a large invasion harbor even by sailing-era standards. Winds, currents, and usable landing sites pushed the French toward defended parts of southern England rather than allowing them to choose an empty coast at will. Dover and Portsmouth matter because they are not just map labels; they are fortified bases that could shelter British naval resistance and threaten French supply movement after the landing. Dover Castle also protects a port close enough to interfere with the invasion's sea line of communication. Harbor capacity and coastal geography constrained the scheme before any battle began.
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Boulogne and the landing geography
A close look at Boulogne, Dover, Portsmouth, and the Channel approaches shows why the invasion route was constrained before any battle began.