Hi OP,
Presumably the university have sent you some info to help you prep and what to expect? Things like the format of the interview, presentations etc? Healthcare course recruitment in general is values based, so look at the NHS 6c's & the NMC ethical standards for nurses and structure your answers / anecdotes around them. Think about using the STAR method for answering questions (Situation, Task, Action, Result) to keep your answers concise and to the point. Keep in mind that interviewers are looking for answers that demonstrate that your own values align with those of the profession, that you understand what the job and the training entails (the good and the bad), they're also looking for good communication skills (essential in nursing). Know your personal statement and be prepared to answer questions about it (revise anything you've talked about or over-egged your skills or knowledge in). Have a look at the nursing times or similar to see what's happening in mental health nursing at the moment, it may well come up in your interview.
Best of luck 🙂