I expect the question the midwife would ask is why it is a necessity. If there is a medical reason, normally she would be willing for you to have this procedure. If you know that there is such a reason and you feel that your midwife is wrongly refusing to allow it, then perhaps it's not really a question of trying to convince her but of getting a second opinion. It may be a good idea to see a doctor or other health professional and explain your concerns. You could tell your midwife that you want to do this, and she may be able to refer you to someone.

If there is no actual medical reason, then you aren't entitled to a caesarian and would not normally be given. Of course it's hard to give any detailed advice without knowing exactly why you want a caesarian, but if you click here you can find a lot more information, including reasons why you may or may not be advised to have one.