Many roles at the NHS require you to work rotational shifts or to be available to work nights, weekends or holidays.
Before answering questions about your availability, you should have an idea of the requirements of the position. If you haven't gathered the information from the job posting or description, now is a good time to confirm what is required if you join the NHS. Consider asking the interviewer what their expectations are; for example, are the shifts 8,10, or 12 hours, etc., and are weekends required? If their schedule expectations won't work for you, consider what you can offer instead and see if you can meet in the middle. It's much easier to discuss these things in the interview to make sure it aligns with both the organisation's needs as well as your own personal needs.