Hmmm...random idea: I wonder if either Agamemnon or Agincourt could be redirected to the RAN and replacements sought? Would allow BAE Systems' yard to keep going longer, would get quicker solution for RAN and would provide pattern for future Australian builds.
So arguably the fastest option for the RAN to get an Astute is one of the Current Build.
But to fill that gap in the RN, more, new SSN production is needed and this cannot be with PWR-2 reactors.
So a modified Batch II Astute with PWR-3 reactor is on the cards....possibly needing a larger diameter hull.
De-risking Dreadnought reactor and propulsion in the process.
There is not enough time now.
Steel for Dreadnought and Valiant began being cut in 2016 and 2019 respectively, its not going to be long before Barrow start assembling hull sections and needing assembly space. BAE Systems seems able to build sections for at least three submarines at once and assembling two.
Currently Barrow has:
Anson - fitting out and on the water since April 2021, probably will commission in 2023
Agamemnon - building since 2013, probably won't launch until 2023 and complete in 2025
Agincourt - building since 2018, is scheduled to complete in 2026 which seems unlikely, most Astutes have taken 11-12 years to build so 2027-28 seems more realistic. A launch isn't likely until 2024-25.
Dreadnought - construction (steel cutting) began in 2016, but presumably assembly must be underway on some modules by now. Given the Astute build times and this is a first of class it seems likely commissioning will be circa 2030, launching isn't likely before 2026
Valiant - steel cutting began in 2019 so is three years behind Dreadnought, probably launching around 2029 and commissioning circa 2033
Warspite - presumably steel cutting will begin in 2023, completion 2036
King George VI - presumably steel cutting begins 2026, completion 2039
SSN(R) 1 - presumably steel cut sometime around 2030 with completion in circa 2044?
So even beyond the PWR2 issue, where could two or even one Astute be inserting into the build programme without either pushing back the last two Dreadnoughts?
Even if the RAN in 18 months time goes for an Astute programme boat and ordered one from Barrow that means steel cutting in 2023/24 and completion in 2035 - which means taking Warspite build slot. Replacing Agamemnon and Agincourt would mean waiting until at least 2036-40 to get a full complement of SSNs again and potentially pushing the last two Successors back into the mid-2040s (way past Vanguard expiry) and pushing service entry of SSN(R) back to 2050.
I don't think this will happen, I think a far more likely scenario given the timescales is that Australia will join the SSN(R) programme -
if it wants a BAE sub - and that SSN(R) will become a multinational programme like Tempest.
BAE could build a first one and have it completed circa 2045 and get the bugs out and UK and Australian built boats in tandem could churn out 8 subs each between 2045-60.
Now the only question mark is how much capacity has Rolls-Royce got to build reactors? It's never had to handle more than 4 within any procurement block, Astute has been spread out since 2001 - 20 years and counting. 7 reactors in 20 years is not exactly a sausage factory... Assuming RR have 20 years (2016-2036) to build the four PWR3s for the Dreadnoughts, is it going to be able to double that build possibly up to sixteen PWR4s for SSN(R) and AUKUS SSN over 20 years? That kind of rate alone might mean a US reactor is preferred if General Electric has sufficient capacity to meet whatever SSN(X) build rate follows the Virginias from the late 2040s.