Re: What the hell?
Hopefully there won't be too many delays. They're already building at least three of the new Dreadnought class at Barrow - it might even be all four - while they finish off the last of the Astute's. Labour didn't build any more boats after the final Vanguard in 98 (the last attack boat was in 91) and Major's government hadn't ordered new hunter-killers either. Astute wasn't ordered until 2001 - so there was gap and a lot of skills/staff were lost. You need a lot of welding, checked incredibly thoroughly, or your submarine goes down but not back up again. And they had to train an awful lot of people to get the production line going again. We'd also lost design skills, and that took time to get back up to speed as well. Thus the first two Astutes were both very late, and very over budget. But by boat 4 the price was down to about £1.2 billion. At the same point a US Virigina class was about £3bn - although they are bigger because of all the extra cruise missiles.
So hopefully the program is on target, and the subs should be in service about when planned.
They need to be, because we need to start working on the AUKUS sub. We only built 6 (+1 finishing) Astute, and we need more. But the reactor design is old, so they can't build any more, and the new PWR3 for the Dreadnought class is too big to fit in. So the new sub has to be a new design, and we need to get one in service as soon as so we can get more built - and get the Aussies set up to build theirs.
I don't know if it'll be a help or a hindrance, but there are Aussie workers and Navy people already training, in order to gain the skills they're going to need in future. So more people going through the training pipeline, but also more people available to maybe speed up the build process of the Dreadnoughts - to get the AUKUS boats out the door.
Oddly although we're building submarines close to capacity, this is artificially limited. Building what you need in a rush and stopping means you get capability gaps and things go wrong. So the sweet spot is to knock out one boat every 2 years, if you plan to have 11 boats, and run them for twenty-odd years. If we wanted to have say 10 hunder-killers and 4 bombers, then a 1.5 year build program would seem about right - giving a slight delay for the change-over between designs and maybe building the first boat for Australia.
You can make the subs last more than 20 years. But if you do, you have to refuel the reactor. Technically you actually replace the whole core, and this is why Vanguard took 7 years to re-fit, because they'd not done that on this design before and it turned out to be very difficult. So it's probably better to run them shorter, if at all possible - and probably ends up slightly cheaper. Older things take more money to maintain.
Japan has the right idea here. They started building their Soryu class submarines at a rate of one a year. As they did with the previous class of 11. Of course they're diesel-electric and so a lot cheaper than nuclear boats. Their plan is just to carry on building at this rate, until they need a new design, and then start building that. They can then either sell off any excess, or retire them early - once they've replaced the older models. I think they currently operate 22 - so I guess that gives them a 22 year life-span, with the option to quickly increase their operational fleet (by keeping them a bit longer) if they think China are about to kick off.