The final boat, HMS Triumph was due to be retired this year, that has been extended 18 months to 2024.
Several of the boats have, as they’ve aged, experienced thermal fatigue cracks in the reactor coolant systems, some critical. The PW2 reactors also don’t meet modern safety rules.
The chances of any having their life extended enough to be offered even temporarily to the RAN are between nil….. and nil.
The current proposal to base a couple of Astute in Australia and dual man them whilst training reactor officers and COs is about as far as the RN can stretch.