A very interesting contract announced yesterday: The UK is paying General Dynamics Electric Boat unit to design a Common Missile Compartment (CMC) for its next ballistic missile submarine (SSN), slated to replace the Vanguard-class SSBNs from 2022 onwards. The contract could be worth almost $600 million.
However, the contract also states that the CMC is intended for the US Navy's Ohio-class replacement as well. While the US and UK SSBN programs have long been joined at the hip through the use of the same missiles, this is another step forward in collaboration. The US Navy, meanwhile, is looking for an engineer to run its end of the project.
The UK decided in 2006 to maintain its deterrent based on SSBNs and started design work in 2007 - Richard Beedall has a very comprehensive page on it here. A major concern has been the maintenance of a nuclear submarine engineering base - a break in construction was one reason for delays in the Astute submarine program.
Meanwhile, the US Navy faces a similar situation, having accelerated the Virginia-class program to reduce costs. To avoid a similar break, the USN now wants to start construction of the first Ohio-class replacement in 2019 (page 3), aligning it closely with the UK program.
Final requirements have not been discussed, but the missile compartment's size and shape pretty much determines the size of the boat. It would not be at all surprising to see the new boats emerge with fewer tubes, probably Trident-sized but adapted from the start to multiple payloads: cruise missiles, smaller conventional ballistic missiles, UAVs and special operations support.