In response to BUREAUCRATUS LEX’s lucid comments of March 3, 2022, I reckon:
Australia’s Prime Minister Morrison has donned khaki of all shades to take to Australia’s May Federal Election. AUKUS, and the SSNs to be commissioned in about 2 decades, are but a portion.
If future Aus SSNs come in the form of Virginia Blocks Vs or VIs, with 6 Vertical Payload Tubes, they will be true SSGNs with SLBM potential to deter the Putins and Xis overseas.
The UK perception of Putin invading Ukraine and Putin threatening a higher nuclear alert will be to increase UK reliance on UK SSBNs and the limited number of UK SSNs to protect them.
Along with SSBN protection the UK’s relatively small force of 7 Astute SSNs (after 2026) have the renewed role of protecting the UK’s Queen Elizabeth-class carrier groups. Seven Astutes may boil down to 4 available given various states of maintenance for 3 of them.
The UK RN has usually considered 10+ SSNs as a bare minimum for their many roles.
So you are correct that it will be 'damn near impossible' that the UK will lease even one of its Astutes for training or part crewing (with RAN submariners still on a learning curve).
Russia’s Ukraine aggression, Putin's nuclear alert and China/PRC's well established territorial aims will increase the US perception that all SSNs built, or part built, in the US should go to the USN.
All this may mean if Australia wants SSNs (or SSGNs) there will be greater pressure to build them in Australia and do the main training on the first Adelaide made sub delivered.
Anti-Missile Systems
The trend of Russia using conventional warhead cruise and ballistic missiles against Ukraine and scope that Chinese nuclear subs could use the same against Australia might justify Australian anti-missile shields. But the dispersed nature of our major cities makes only partially effective shields (like AEGIS Ashore) even less effective. Small land area countries, like Japan and Israel, can effectively use anti-missiles but unfortunately Australia isn’t small.
Also Aussie F-35s would make little impression on Chinese subs. The best ASW platforms are SSNs and we’ll need to wait about 2 decades for ours to be effective and available in sufficient numbers.
Nuclear Deterrent
The probable 20 year hiatus until these Aus SSGNs actually come into service will maintain Australia’s over reliance on the US nuclear umbrella. The aging Collins can backup US operations but do little to deter the Chinese and Russian nuclear superpowers.
The best thing to give our nuclear armed (potential) enemies pause, is a nuclear deterrent, preferably under US control. But if the US is led by Putin’s erratic mate Trump and by like-minded future US Presidents, then preferably nuclear weapons under Australian control.