The interviewers ask this question to see that you have read the job description in its entirety and determine what aspect of the role you like the most. Most NHS Band 7 physiotherapist job postings have many job duties you will be asked to perform. The interviewers want to know what you like the most to see where your strengths and interests lie.
Describe what you like most about the job and provide your reason why. For example, you might like that you will be responsible for training and supervising junior physiotherapists. Tell the interviewers why this particular job duty excites you.