Petition Policing; Disregarding Mental Health

The policing system here in the UK does not have sufficient training to deal with cases of mental health. The police look for evidence, evidence that is typically observable.
True, anyone can claim they are not mentally stable, but those with treated disorders will have documentation stating their diagnoses from a psychiatrist.
Police should make it clear: they may have representation from their social worker, through a solicitor. Recently, disregard of this left one case alone, vulnerable, degraded and their mental health issue being described as an 'excuse' and 'load of rubbish', despite the individual being heavily medicated and having letters to prove such.
In short, the police need more mental health training. They must not 'look down' upon those who are equal but have just happened to do something wrong. Officers who disregard this should not have their job.