Well, you *can* look behind you, but as Sally says, only on one side, so that will score you at very least a minor fault for 'lack of all-round observation'. So, if you're going to have to use the mirrors on the nearside of the car anyway, why not learn to use them both? For one thing, it prevents your brain from getting confused by having to keep switching views from real to mirror image all the time.
One problem I had was that I'd learned to reverse by not only looking out of the window, but also looking in the rear view mirror, and *over* the trailer. Of course the moment the trailer gets bigger or becomes a horsebox, I'm well and truly stuffed, and my visibility is reduced to one side of the trailer only - not ideal!
None of it is hard though - it just takes practice.
BTW NethertonSH, local knowledge is definitely a big advantage. I thought the examiner told me to follow signs to "Torfitshen", then got all flustered when I couldn't see any. In the end I went to Torpichen, as it was the closest match I could find. That seemed to work well enough!