Physical restraint should always be used as a last resort ad to keep people safe, not as a means of making a child co-operate or ensuring they follow instruction.
"My concern is that there are teachers and other school staff who are inadequately trained in managing special needs who allow situations to get out of control. If random and untrained physical restraint is allowed (or even in some circumstances encouraged) more children like my son will be harmed and left with long term damage. Surely that can't be acceptable by anyone's standards."
My concern exactly. Training in behaviour management is essential to avoid situations such as this; for both the students' and the teachers' sakes. Children with SEN, histories of abuse and emotional difficulties are particularly at risk. Teachers are leaving themselves open to physical injury and damaged relationships, even leaving aside the issue of legal action.
(I also think that you should be able to train as a SEN teacher, but that's a whole other argument)