I would not go into OR if you do not believe the doctor will follow your wishes.
Here are a couple things to consider. Some doctors will believe that having a fibroid is a medical condition that dictates the removal of the cervix. Why? The cervix is not a separate organ. Instead, it is the neck of the uterus. As such, fibroids can grow from the cervix both prior to and following a partial hysterectomy.
Some doctors do believe that age dictates the removal of the ovaries as they stop functioning--though they do still produce a trace amount of hormones. Thus, age alone could be for them a medical reason to remove the ovaries.
Anything you put in writing would have to include so many if this, then thats, and state which medical conditions you don't feel are ones for which you want the cervix and ovaries removed. It could be very difficult to cover all the what if's. It would be better to have a surgeon who is on the same page as you right up front.
I would suggest sitting down with your GP and discussing the situation. That doctor may be able to help you write something that would cover at least almost everything. However, I would strongly recommend a second opinion, and your GP may be able to help refer you.