Ask an Immigration Solicitor. Get an Answer ASAP.
Thank you for your enquiry.
You have options here.
Option 1 apply to renew your South African passport, which will allow you to travel out but you must also travel with the old passport containing your ILR. OK you may also need a visa to visit Australia.
Option 2 apply to naturalise as British but the process can take up to 6 months or more. Once you have been naturalised you can then apply for a British passport. The whole process can take time to complete from start to finish so may not be the appropriate option if time is of the essence.
If you have already done the Life in the UK test as part of your indefinite leave to remain application then you don't need to do it again.