As you've effectively got two threads going on broadly the same topic, Jennyjoan, I'll try to cover them both here:
As I wrote previously, the only way to get from Sydney to Belfast with just a single change of plane is by changing at Heathrow. The total journey time is around 27 hours (although it takes somewhat longer getting back). It's too early to book flights for October 2021 at the moment but, based upon fares currently being offered for next May, here's a rough guide to what BA would charge for return flights:
Economy Standard Class: £800
Premium Economy Class: £2,100
Business Class: £3,700
First Class: £6,700
You can tell by their prices that BA obviously aren't too keen on allowing riffraff outside of Economy Standard ;-)