Working in the healthcare industry can be a 24-hour around the clock job (depending on your role), as people are constantly becoming sick and injured. If working weekends and holidays is part of the role you are applying to, the NHS should have mentioned that in the job description, so it should not come as a surprise. The interviewers want to ensure you understand these requirements and that it won't be an issue for you if hired. Show them you are flexible, a team player, and will work what schedule is best to fill the needs of the NHS.