Police officers are not trained psychiatrists and, if faced with physical attack, do not have the expertise, or the luxury of time, to determine whether their attacker is mentally ill.
The use of CS spray on NHS premises to "quieten patients" highlights the inadequacies in primary healthcare provisions for the mentally ill. Presumably, there is nothing to stop such hospitals refusing police permission to use CS spray on their premises.
The death of PC Nina MacKay (fatally stabbed by a paranoid schizophrenic last year) served further to illustrate the dangers police officers can face when dealing with dangerous mentally ill offenders. Certainly CS spray is not appropriate in all circumstances, but its use is preferable to possible longer term injuries and fatalities.