I'm interested in studying medicine at university. I hear that the "Why do you wish to study medicine/become a doctor" question is asked a lot at interviews. How do I go about answering it on the spot?

In a similiar fashion to certain topics and/or questions that you would most likely expect to come up in an exam, you should always prepare well beforehand. For such a specific and common question asked in medical school interviews, it can be difficult to know what to say given the widely varying columns of advice an A-level student is bound to be exposed to.
Whilst common answers involving notions of 'wanting to help people', 'interest in science/human body' and other cliche responses are completely valid and may, in fact, be the primary reason(s) many people choose to pursue a career in medicine, they instill the usual sense of boredom for medical school interviewers. Instead, although supplementary reasons may not necessarily need to be original, interviewers must get a sense of the ‘personal’ and well-thought out nature of your decision to grind out many years of medical school in order to pursue such a profession. Therefore, showing that you know what the job will entail, including its negatives, from medical work experience or talking to doctors in general will demonstrate a mature, conscientious outlook to the interviewer.