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).