Andrew Hastie may have a military background but, he doesn’t know what he doesn’t know.
The PWR2 reactor doesn’t meet modern safety requirements & the PWR3 is reported as not being able to be modified to fit the Astute hull. Nuclear reactors require a lot of lead time to process the fuel and certainly can’t be done in the timeframe that he is indicating. The PWR2 Core H only has a 25 year life in comparison with the S9G reactor’s 33 years & the Virginia can cruise with just convective reactor cooling whilst the Astute needs to run coolant pumps at all times. There’s a huge difference in performance and capability of these two submarines.
I suspect his comments are of a political nature aimed for widespread consumption of the masses (& possibly to establish political friendships in the UK).