My suggestion is to book a one way ticket to the UK. When it's time to return, pray that the situation has improved (both for your father and covid) and book another one way ticket to Australia based on whatever is available then.
I'm an Australian PR based outside of Australia. We get weekly updates from the embassy about flight availability going into Australia, and as @
kiwifrequentflyer mentioned, seats open up depending on available quota slots. It's easier to get a seat as an individual and SQ tends to have slightly better availability but tickets are typically about 2x more expensive pre-covid and that's not taking into account the A$3000 quarantine fee