You have more than enough points for two return business tickets from Australia to the UK, including on partner airlines such as Qatar who costs a bit more in points.
Qantas releases its award seats 353 days before departure so that is the best time to look (i.e. June 2016). Given it is the same amount of points whether you book a one-way or return, you should search for your UK-bound seats as soon as the award window opens, then do them same for you return. Don’t worry about trying to book them as a single ticket.
As long as you can be flexible by a few days, I’d say you have a really good chance of getting two business seats. Just make yourself familiar with the Qantas search engine and then keep checking multiple times a day when it is 353 days out.