The NHS wants to hire a radiographer who is responsible, dependable, and shows up to work when scheduled. They do not want an employee who does not show up to work or calls off at the last minute, leaving the radiographer you will relieve and the department in a pinch. The interviewers ask this question to evaluate your work attendance in the past year. If you called off work many times in the past year, they might hesitate to offer you the position unless your reasons were excusable.
Give the exact number of days you missed work and explain why. If the interviewers deem your absence from work an excused and unavoidable reason to miss work, it will likely not be an issue. If you have many unexcused absences, they may disqualify you as a viable candidate.