Noting the Astute replacements are known as "SSN(R)" and "SSNR". The
replacements are "expected to replace the Astute-class during the 2040s".
The last Astute, HMS Agincourt, is expected to be commissioned in 2026.
So Australia buying UK SSNs would fall between the Astute-class's actual production
availability and the Astutes don't have the fairly important VLS (for AUKUS
hypersonic cruise missiles).
When the almost certain US Combat System integration for Australia's AUKUS SSN
is included the US Virginia class better fits Australia's content and production schedules.