If you get involved with the police - perhaps as a witness or victim of crime or you are detained, arrested and/or taken to a police station - you have the same rights as anyone else.
If you are deaf, have a hearing impairment or speech difficulties the police should arrange for an interpreter to be present. They should not question you until the interpreter is present unless a delay would mean an immediate risk of harm to someone or serious loss of, or damage to, property.
The police should only interview someone who has a learning disability when a responsible person (referred to as an 'appropriate adult') is present. This person should not be employed by the police and should be experienced in dealing with people with learning disabilities. It could be a relative of the person who is interviewed or someone responsible for their care.
If you have a learning disability, the police should not interview you until a responsible person is present unless delay would result in a risk of harm to property, people or evidence connected to the offence.
If you are detained, you are entitled to a medical examination by a healthcare worker, for example a paramedic, a nurse or a police surgeon - sometimes know as a Forensic Medical Examiner. You may also be examined by a general practitioner (GP) that you choose, if they are available. You may have to pay for this, however. This will be formally recorded.
The charity Mind have a useful factsheet relating to the police and mental health. This factsheet is available as a webpage and in PDF format on the Mind website.
Part of the Mental Health Act 1983 (section 136) details removing a mentally ill person from a public place to a place of safety. It details police powers and the rights of someone in this position.