"It is acceptable to adjust the mirrors before a manoeuvre on a driving test if it is to make the manoeuvre safer by providing better observation. Although this is acceptable, it is perhaps not advised due to a learner forgetting to re-set the mirrors when the manoeuvre has been completed."
Besides, I think if you do the bay parking manoeuvre, it will be at the start of the test, so point your mirrors down before the test, and you will be ready for it. If he says "pull away and take a left" or whatever, then while the car is still secured you can adjust them for normal driving.