I think the 50,000 may be an EU standard.
How does one define a cathedral, btw? If it means "any church that the Church of England have given a cathedral status to", then it just means that city status can be bestowed (or revoked) by the CoE, in theory more or less arbitrarily (though, in practice, the CoE's hands are somewhat tied by the status quo and public expectation).
What's the Australian standard for what constitutes a city, btw?
That's interesting. In England, it's the presence of a cathedral.