I've been at Cammell Lairds a few times, on a couple of occasions they have been in the midst of getting Astute hull modules ready for delivery. The last time I was there they had the bow for no. 7 in the building hall.The figures for Astute are a little misleading. The start date listed is really just procurement of long lead items. When I was there around 2012 there was not much activity on boat four.
As others have said the reasons are the need to maintain continuous build, plus, in the past, the restricted ability of Rolls to mass produce PWR2, which was at the end of its development life and becoming a limiting factor on the program.
If Australia go down the "Astute +" route there's the opportunity to bring it up to a much more economic level of activity and cut the build times.
But no-one is ever going to get close to the US, which is a completely different model, more like a car factory. They bulk-procure material safe in the knowledge that there's another five boats coming up soon, it would only work in the US.
If Australia build Astutes surely they would also follow the same time line as the UK, when the final hull is out the door the first successor hull will be entering production.