In the Lord of the Flies why are the british boys alone on a deserted tropical island?

The boys don't 'fight' in Lord of the Flies. Quite simply the boys elect Ralph as their leader and Jack, the head boy of a choir, resents this and later tries to have himself elected as leader to replace Ralph. When this fails he eventually goes off alone and over time almost all of the otehr boys join his 'tribe.' Jack realises that while Ralph remains on the island he will always pose a threat to Jack's leadership, because Ralph was the rightfully elected leader, so Jack decides to hunt Ralph down and kill him.