It is very important that the boys are British.  Golding includes several moments in the book that involve the boys discussing how the British are best at everything, or how British adults would behave, etc.  First of all, it allows for even more of a downfall when the boys' inherent savagery emerges because, like it or not, we're not expecting this behavior from a group of private school British boys.  Even the shedding of their uniforms piece by piece is symbolically effective (more so than if they were not wearing uniforms to begin with).  Secondly, Golding is making a commentary on the British involvement in war during this time period.  he is potentially poking some jibes at the British and at the assumption that they are the best at everything, always remaining proper and dignified.  Golding's own experiences in the war taught him a lot about the inherent evil within us all.

I don't know that it is important that the boys are British. The book was first published in England, but other than that, the theme of the novel can pertain to any nationality.

The theme is the inherent evil in man and the brutal capability that lies within each person. It is also about the importance of rules in society and what would happen if that structure were to be lost or thrown away. This is shown throughout the novel as the boys progressively embrace their inner beast and dismiss the laws of society.