From the perspective of personnel evacuation, Dunkirk could hardly be considered an appropriate port due to its shallow waters not being suitable for large ships. Instead, small boats had to shuttle back and forth between the large ships anchored offshore and the port, leading to inefficient transfers. If ports like Calais, Brest, or Antwerp were involved, large ships could dock directly and carry passengers to Dover on the opposite shore.
However, smaller ports have their advantages, such as being less conspicuous. Another advantage is that even if spies are watching, it's difficult for them to determine which small boats are meeting which large ships and where those ships are headed. Using familiars or aircraft to scout the area would not only violate Albion's sovereignty but also expose their identities.
From the perspective of secrecy and evacuation efficiency, this port was the best option available.
