[Joppa]

As things stand (Brexit will almost certainly change everything), unmarried partner is regarded as extended family member and needs to be in a durable relationship, usually meaning continuous cohabitation for 2 years. Or just get married. EEA law only legislates about married couples. Definition of unmarried couples are left to member states to decide, and UK has applied the same criteria of 2-year cohabitation for those under EEA regulation as well as UK immigration rules. You may get approved for less than 2 years, but I wouldn't count on it, as Home Office is hardly likely to make things easy while Brexit talks are under way.

How your situation will change after Brexit is anyone's guess. Even if you get married, and obtain residence card under EEA rules, Brexit can mean you still need to get authorisation from Home Office to remain in UK. What that entails, and what conditions are attached, are unknown. It's possible that you may need to meet the same or similar financial requirement as under the family route (£18,600 etc). You just need to wait and see.

 

[Joppa]

Maybe, but I can't imagine Home Office to be in a generous mood when Brexit - and taking control of EU migration - is in the offing. Rather I'd expect them to stick to the letter of the law.
Why don't you get married?

 

[Joppa]

Nowadays being in the Commonwealth has few if any advantages. The only thing I can think of is UK Ancestry which allows you to live and work in UK for 5 years, for which you require a Commonwealth citizenship and a grandparent born in UK, i.e. England, Wales, Scotland and Northern Ireland (or in the Irish Republic prior to 31st March 1922), plus Channel Islands and Isle of Man.
I don't think being British confers more than a few rights in Australia.

 

[Joppa]

History means little and present political reality a lot more.