A Your information is not strictly accurate. There are instances when it would be within your duty of care to minimise risk, and you could be charged with negligence if you failed to act in a reasonable way, including use of appropriate physical restraint when people are in danger if you do not step in.
Your local education authority and school should have policies detailing their positions under current legislation and the protocol staff are expected to follow. The main thrust is to focus on avoiding confrontation (de-escalation, and strategies for following-up and moving forward after a crisis).
Physical intervention is a daily occurrence in certain settings, but because it is less frequently used by mainstream staff they are less likely to have received specific training. This is why they are most at risk of causing harm to themselves or to others by intervening unsafely and failing to document what happened and why they acted as they did.
Raise this with your senior managers. There are excellent training packages around, and some authorities have developed "in-house" versions and tutoring systems.
* Please email questions to SNExtra@tes.co.uk or write to TES Extra for Special Needs, Admiral House, 66-68 East Smithfield, London ElW IBX