Why does the Royal Navy perform many tasks that belong to the Coast Guard in the United States?
Short answerBritain has one navy rather than a separate armed maritime service, so the Royal Navy absorbs many Coast Guard-like functions.
NotesThe difference is institutional. The United States maintains a distinct armed maritime service with its own cutters and enforcement role. Britain assigns a wider range of constabulary and security work to the Royal Navy. Those duties also give the Royal Navy additional peacetime missions through which to demonstrate its utility.