Ask H&H: speed limiters

Q: WE have a five-tonne, two-horse Iveco Daily horsebox that was new in March 2006 and had its first plating (the HGCV annual test) in March 2007. It is now due to be plated again, and I have been told it will not be considered for plating until a speed limiter has been fitted, limiting the speed to 56mph. I rarely drive at this speed — unless on the motorway when I tend to keep to 60mph. Is this requirement limited to certain boxes or does it affect them generally?

DS, Dorset