The movie begins with the boys in an airplane. They were being evacuated and moved out of the area. During World War II, London was devastated. Buildings were destroyed, and it makes sense that they would try to evacuate for safety. The idea in the movie that something nuclear took away all of the power of planes in the air makes sense for them to crash where they did. Although the book discusses the fact that they were shot down, that makes sense because it was during the/a war. So it was pretty effective for them to begin the movie explaining what was going on in the world around them before they landed on this island.