Hi!
Just finished reading this book, so I can help.
Somewhere in the book, Piggy mentions an "atom bomb" and we know already they were in a plane crash. With the fact that they were a bunch of boys in a plane, and mentioning of an "atom bomb" we can suppose they were escaping from a war of some type. In the book, they are also portrayed as British through their dialect. All in all, they most likely were evacuating Britain because of a war.
