XLUUVs as a littoral complement to nuclear submarines

What are XLUUVs, and why would the Royal Navy want them?

Extra-large uncrewed underwater vehicles offer a smaller inshore complement to growing nuclear submarines, with conventional propulsion proposed to provide endurance without an unattended reactor.

The 2021 discussion treats extra-large uncrewed underwater vehicles as a response to nuclear submarines becoming too large for some inshore work. A separate crewed conventional-submarine program could compete with SSN funding, whereas uncrewed vehicles offer another route to that capability. Diesel-based propulsion is proposed as a way to obtain the desired range. Nuclear propulsion is rejected because an uncrewed reactor could operate far from communications and supervision. This is an interpretation of the developing role and its budget incentives, with the machinery choice still discussed as a proposal.