Britain has hired an American submarine builder (General Dynamics) to design a Common Missile Compartment (CMC) for Britains next class of ballistic missile submarine (SSBN), which are to begin replacing the current Vanguard class boats in 2022. The assignment specifically mentions that the U.S. Navy will use the CMC for its next class of SSBNs. This makes sense, because Britain buys the ballistic missiles for its SSBNs from the United States. It would be too expensive for Britain to design and build its own SSBN ballistic missiles. Thus the CMC will have to be designed by an American firm, with access to data on the characteristics (especially the dimensions) of future missiles for SSBNs.
Britain and the United States have long cooperated on designing nuclear submarines, especially SSBNs. The U.S. and Britain are designing two different SSBNs. But each sub will have many common features, like the CMC, and that will save a lot of money for both nations. The 18 U.S. Ohio class SSBNs were built between 1979 and 1997. The Ohios were built to serve for twenty years, but that has been extended at least 15, and possibly 30 years. In the next 5-10 years, work on a new class of U.S. SSBNs will have to begin.