The references to PWR3 being based on US reactors is misleading. Direct quote from source
"The Ministry of Defence is now designing the PWR3 power plant for the successor to the Vanguard Class that will come into service in 2028 and remain operational until at least 2060. This offers the next opportunity for a significant step change in the design of the submarine PWR.
Support from the US has helped us to innovate design in PWR3 to the highest international technical and safety standards, whilst, in addition, training UK designers and engineers in US advances in reactor plant design. While the existing PWR design continues to meet all of the required safety standards PWR3 can now be designed to be even more efficient, easier to operate, better value for money, and to set new standards of safety."