The Navy will snap your arm off if you want to join as a medical officer. TSR is not the best place to ask, the navy have a dedicated medical recruitment team who are extremely experienced and will have answers to all of your questions. I recommend you call them, they will also put you in communications with a senior medical officer who will help answer any questions/doubts/personal insight you have. All your questions will be answered definitively within 3 minutes if you give them a call, and you can even start the application process.
Any reason why you want to join the Navy and not Army or RAF?