The British Ship was a ship commandeered by Captain Moreland and his crew that served as a means of transportation for him to travel back to the island where he was once held captive along with his granddaughter, Roberta (formerly), by Kuala and his pirates and to fire cannons at them, thus destroying them along with their ship and lifeboats in Swiss Family Robinson. It was equipped with cannons used for firing at enemy ships and lifeboats for allowing them to sail to shore.
In the film
Captain Moreland used this ship to travel back to the island where he was once held captive along with his granddaughter, Roberta (formerly), by Kuala and his pirates after the pirates returned him back to England for ransom and to fire cannons at them, thus destroying them along with their ship and lifeboats when all hope seemed lost for the Robinson family since they had almost run out of defenses against the pirates.
Later, Captain Moreland returned back to his ship to take Ernst back to England to finish his studies in a university (which Captain Moreland had been making donations to for years) since there had been a slight change of plans: Father and Mother decided to stay and make the island, renamed New Switzerland, their new home instead of going on to New Guinea as originally planned, Fritz and Roberta decided to stay and make New Switzerland their new home as well and get married instead of going back to England with Captain Moreland as originally planned, and Francis decided to stay as well either way and watch over his animal friends.