In the old days, ship designers sat at big angled desks and used pencil and paper to design compartments. In those days we built plenty of ships and had plenty of people wanting to be ship designers (because it gave them something to do).
Nowadays we don't build many ships, we have few ship designers and everything is designed by CAD/CAM. Resources who know how to use this technology are in limited supply and of those, only a small fraction know how to design ships.
Based on what happened to Astute (and the Kipperbomber) MoD is understandably nervous of something horribly expensive being found half way through the build process.
What Adm West talked about last years was raising the maturity level of the design and reducing risk. This is why it is taking so long for anything to happen.