I assume you are applying for a Submariner role....?
If that is the case expect to be asked about your training and career pipeline, particularly with regard to the stages after you seperate from your general service colleagues. Expect to be asked why you are applying for submarines rather than the surface fleet and some submarine specfific naval knowledge.
For example, the difference between SSN and SSBN roles, where they are based, what weapons they carry etc. It would be logical to assume a focus on Astute might be a possibility but a general overview should be sufficient for the sift.