Not sure what spare UK capacity there is.
As previously noted Barrow is fully booked for Dreadnaught and then SSNR. With slippages they are already having to consider refuelling the early Astute boats to last until SSNR arrives, which poses problems, as they found with the current Trafalgar boats, as they weren’t designed to allow it.
SSNR will use the same reactor as Dreadnaught and is mooted to, effectively, be the same design with the missile module removed and replaced by a UUV dock. It is also mooted, as with the new carriers, to be highly automated in weapon handling etc to enable a much smaller crew than the Astute class even though it will weigh in at over 9000 tons.
The RR Derby reactor plant claims to be on schedule with PWR3 production and eager to provide the RAN with reactor modules.Which would also get around the ossicle export problems with a US reactor.
https://www.express.co.uk/news/scien...t-news-defence
The only feasible UK based route would seem to be for the RAN to take the SSNR design and, instead of removing the Trident missile module altogether, replace it with a VPM - and built it in Australia to create and sustain the required parts and maintenance base.