Why is this so confusing? Just use the pen you are given. My school didn’t allow us to bring in anything, they provided a clear pencil case with a pen, pencil, rubber, protractor, calculator, and probably something I’ve forgotten, for every exam. Obviously non-calc papers we had to put it under our chair and they checked. We didn’t get the choice of what we are comfortable with, it was their cheap black biro or nothing. If it ran out (they often did as they didn’t replace them after each exam) then we asked for a new one mid exam. If my entire school can manage with a cheap black biro in all of our exams then sacrificing your favourite gel pen for a nice biro is no big deal. Don’t try and cheat it and lose marks.