As one who works in the UK Patent Office which administers copyright I think the registered letter method would be enough to establish the date of your work and perhaps your copyright. But,remember, that you would have to sue any infringer in the courts and that ain't cheap (which won't come as a surprise). Sorry to be pragmatic but the first thing you should think about is "will it valuable enoughto make this worthwhile?" I'm afraid that in most cases the answer is "NO". If it goes on to be a million seller and you lose out on your copyright I shall, of course, deny any of this advice.