William Golding's classic novel Lord of the Flies was greatly influenced by his occupation as a British schoolteacher at Bishop Wordsworth's School in Salisbury, where he taught English and philosophy. As a teacher, Golding experienced firsthand the unruly nature of British boys. Throughout the novel, Golding examines the inherent wickedness present in each individual by illustrating how a group of civilized British schoolboys develop into savages after being abandoned on an uninhabited island. Golding's decision to use only British boys as characters throughout the novel was intentional and reflected the reputation of British children as being particularly moral. Great Britain was considered a world leader and had the reputation of producing polite, honest children, who would eventually inherit the austere perception of their respected country. Essentially, Golding was trying to portray how evil and wickedness is inherently present in the most reputable, innocent people. The well-mannered, civilized British boys descend into savagery and become brutal, malevolent individuals. The transformation of the boys is shocking and thematically illustrates Golding's ideas regarding humanity's sinful, debased nature. 

Here's why: British school boys are first, British. The stereotype of the British is that they are highly civilized and play by the rules. Golding might argue that they are therefore the least likely to fall victim to primal urges and become savage. He chose school boys of various ages, but all under the age of recognizable moral maturity because he wanted to present the argument that without societal constraints, we are all savages; if he had used adults, adults would behave more rationally--at least for a while. He also excluded girls because, in his own words, he did not want "sex to rear its ugly head." He wanted nothing to interfere with his vision of the basic instincts of humanity.

One can assume that the main reason Golding used British school boys is that he was himself British. He had attended the sort of schools that these boys were attending, and he was a schoolmaster in the British system. He had been through this system that was supposed to produce civilized young men, and he was now part of it, and both perspectives gave him lots of chances to observe just how wild and uncivilized these boys were, and just how close to the surface their basic savage nature could be found.

In a general sense, the British were supposed to be civilized, and had for many decades thought of themselves as carrying civilization with them as they went into foreign lands and ran their empire. The book is an extended commentary on this ideal.

For more on Golding's background, see the enotes biography on him (available via the link below).
Greg