There is no requirement for you to display rear marker plates if your vehicle is under 12 tonnes. (& I think trailer under 10 tonnes) If they exceed this, then yes, they must be displayed. If under, you may display them if your total length is 7.5M or longer. If you do display them, then the road rules allow you to make the wider turns, either left or right from the lane or part of the lane next to the one closest to the kerb, if that all makes sense.