Feelinggood, of course you are allowed to touch them- why on earth would you think that you weren't?
As teachers, we can restrain children if:
-A child is injuring others
-A child is injuring themselves
-A child is damaging property
-A child is behaving in a way that is likely to disrupt good order.
Any physical intervention taken must be reasonable, proportionate and necessary.
This means that if a pupil is charging at another holding a knife, you are justified in rugby tackling them to the ground- if they are verbally abusing you, you are not.
The vague definitions do not help- 'damaging property' could mean smashing a window; it could also mean snapping a pencil in two. Legally, you could restrain a child for this, but ethically? It would be completely inappropriate. The legislation relies on teachers' judgements of what is reasonable, proportionate and necessary, and there is rarely any provision to train them to effectively make that kind of decision.
As a teacher, you have a duty of care to keep children safe, and act in loco parentis.
This does not mean that you are required to restrain a child, but that you cannot do nothing. Even fetching someone else to help is something.
So, leaving aside a number of non- physical interventions which may have worked with this particular child, staff could have restrained him and/or sat with him until he calmed down. There is no need to seclude a child in such a way unless (possibly) it was an emergency reaction to a dangerous situation. Given the information we have, we can safely assume that it was not an unplanned reaction.