shouldn't 404 be "four hundred and four"?

Yes, it most certainly should, at least in British English (which the OP explicitly refers to). Similarly, 404404 should be "four hundred and four thousand, four hundred and four".

Furthermore, there has been no difference in current US and British English usage for 'big' numbers (billion, trillion, etc) for at least 30-40 years. So when you read "three billion dollars" in the British press, it means exactly the same thing as if you had read it in the North American press.

Trust me. I have been doing financial translations for international banks for more years than I would like to admit to.

Last but not least, 'milliard', 'billiard', 'trilliard', etc do not exist, at least in common usage (except the second in terms such as 'billiard ball' :). A quick poll of my English-speaking colleagues (all specialists in language|numbers|IT) shows that nobody has ever heard of any of them. And indeed, when they figure in dictionaries, it is always with a mention such as "obsolete" or "'Milliard' is not used in modern English".

dave