You need Further Maths for a Physics/Maths degree, or you *will* struggle for offers. Currently studying both concurrently, I can safely say that Further Maths AS is certainly *far* more difficult than Maths AS, although neither are as hard as people make them out to be. Frankly, diversity is meh. If you really want to show diversity, take a foreign language, otherwise I'd advise just sticking with what you know. Wth is World Development anyway?
Saying that, take subjects you're interested in, at least mildly, or you will HATE the A Level.