'Natural person' just means a human as opposed to a company since companies are bound by the law too.
Citizenship is pretty much irrelevant. It's where a person is that determines what law they're under. So if a U.S. Citizen GB member was in Australia, it'd be the same deal as for (I assume) Australian citizen Jackson. It's the fact of a GB member being within the jurisdiction of the RC more so than him being Australian.