If the piece is not listed then you have to ensure it has all the elements in it as described or laid out in the syllabus. As you are trying to find its grade worth then it is a piece you have chosen. Do not aim to high over the bar when choosing any "self choice" piece. Examiners are not there to be impressed, they are there to judge you on what you present, they take marks away and mark down to arrive at your score.
So pick wisely and take your teachers advice on choice, I have seen students with the ability to pass.....fail, because they choose a piece that was just to difficult, and in the pressure situation of playing it to be judged failed to come even close to their true ability.
Playing it and practicing it is not the same a performing it, in performing it you can allow your focus to shift, and before you know it you are making mistakes all over the piece.
Also a little more detail is needed when you make such a post....context is everything
Depending on what Bach Seranande the movement has been taken and transcribed for bass, there are many choices, and the nature of the exam that is to be graded for, makes trying to give a valid answer easier.