What was the author trying to say about “we’re not savages, we’re English”

This statement by Jack is ultimately one of situational irony. The statement hearkens to the general self perceived grandeur and better-than-you mentality of the British upper class. The irony arises with Jack's descent into savagery, which could parody Britain's aparthied in South Africa, or their ruthless control of India. These are specifics that are timely for the date of publication of the novel, but it could also be less specific and refer instead to Britains general xenophobic reputation. Hope this helps you.