The Channel Islands are a group of islands just south of the Hillsbrad Foothills to which the Zul'Dare region belongs. The Horde destroyed a human outpost on these isles to secure a foothold and set up a base to attack the local Alliance coastal towns. The Alliance eventually used transport ships to reach the islands to destroy the base on Zul'Dare.
As the Channel Islands are only mentioned in the mission briefing to Attack on Zul'dare with it being the "Zul'dare region of the Channel Islands", it's possible that the Channel Islands was meant to include other neighboring islands in Warcraft II Crestfall, Kul Tiras and Tol Barad.
Other islands of the channel, situated near Hillsbrad.