It's just the problem getting over on how sensitive the submarine service is in terms of sharing information and allowing foreigners aboard. Think about how much is still classified from WW2 let alone early Cold War. The US is probably VERY hesitant to work with even the closest of allies. I always wondered what the British had to do to get the Poseidon missiles on their subs.
The RN never received the UGM-73 Poseidon C3 SLBM; they got the A3 version of the UGM-27 Polaris missile after the 1962 Nassau Agreement and 1963 Polaris Sales Agreement that I mentioned earlier in this thread. The UK Polaris missiles were US-made, but had UK nuclear warheads and were maintained in the UK. At the time, Polaris A3 was the newest US SLBM. When the US upgraded to the Poseidon C3 in the early 1970s, the UK attempted to acquire it, but relations had soured slightly between the US and UK and the UK instead pursued Chevaline, which replaced one of the Polaris's three reentry vehicles with a decoy system designed to confuse antimissile defenses.
In the late 70s/early 80s with the replacement of their
Resolution-class SSBNs on the horizon, the UK announced an intent to pursue the UGM-96 Trident I C4 missile that the US was incorporating onto the early
Ohio-class SSBNs and backfitting onto some of the older
James Madison and
Benjamin Franklin class SSBNs. When the US announced shortly thereafter that they would be developing the UGM-133 Trident II D5; the UK was offered the D5 instead (this was a mixed bag; while it was a more capable missile it was arguably more capable and more expensive than the UK needed, balanced by the UK desire to have commonality with US SSBNs). The missiles carried on the current UK
Vanguard-class SSBNs are actually maintained by the US, unlike Polaris, and are part of a joint "pool" of missiles maintained by the Kings Bay Naval Base in Georgia. The warheads are UK-manufactured and fitted to the missiles in the UK.
Bear in mind that before all this, the UK nuclear sub program was built on cooperation with the USN; in the 1950s they did ASW exercises with USS
Nautilus and were impressed with the capability, but Rickover being Rickover he initially refused to allow the First Sea Lord (Lord Louis Mountbatten at the time) to even inspect
Nautilus. Mountbatten however was on good terms with Arleigh Burke, the USN Chief of Naval Operations at the time; not only did they manage to change Rickover's mind on giving the UK access to US reactor technology but rather than Rickover's offer to give the UK the S3W reactor design found on the
Skate-class subs, Mountbatten came away in 1958 with the brand new S5W reactor as found on the
Skipjack-class and all US nuclear sub classes up until the 688s. The first UK SSN, HMS
Dreadnought, was basically a US-built S5W reactor and engineering/propulsion section mated to a UK-made forward section, a model I expect the eventual Australian SSNs to follow. The 1958 agreement, unlike AUKUS, also transferred enough technical information to allow Rolls-Royce to design and build their own PWR-1 submarine reactor, which was used on all subsequent UK nuclear subs up until the current
Vanguard and
Astute classes.
To add to my earlier point, the USN at least already sends its bubbleheads to work with the RAN; I had forgotten that the husband of one of my college classmates did an exchange tour with the RAN's
Collins-class subs in Perth. The RAN's current sub fleet was refitted to use the same AN/BYG-1 combat system as the modernized 688is and the
Virginia-class (previously it was the same CCS Mk2 system as the older 688s) and they use the current mods of the Mark 48 ADCAP torpedo (the current Mod 7 was actually co-developed with Australia), so they already have a lot of commonality on the "business end" of the subs.