When a sudden or unexpected death occurs then the police will attend. You should not be alarmed by this.
They have two roles.
Firstly they have to investigate to see if there have been any criminal offences committed. In the case of any drug-related deaths that will involve trying to find who supplied the drugs.
If the supplier is found then the law is that they cannot be charged with manslaughter unless they have helped to administer the drugs. You can expect any sentencing Judge for the supply offence to deal with the case more severely though if a death is involved, and you can expect the police to keep you informed of the investigation and when any court case may take place.
Secondly they have to act as Coroners officers and supply information to the Coroner. This is likely to involve the taking of written witness statements in which you will be asked to provide details about the person who has died. This is likely to ask you about any drug taking that you are aware of along with personal details.
A family member will be asked to formally identify the deceased in front of a police officer. This can be done where the death occurs or may be done later at the hospital mortuary. This cannot be avoided, it has to be done, but you will be treated with sympathy and courtesy. It may be done by any family member, and can be arranged to fit with a final visit to see a loved one.
The police will arrange for the body to be taken away, and this will be to the hospital mortuary as the Coroner will require further investigation. They will use a local undertaker, but this does not mean that you then have to use the same undertaker for any funeral arrangements. If you do decide to use a different undertaker then make sure that you tell them that the Coroner is involved when you contact them.
In both cases the police officer should leave their contact details. Do not hesitate to contact them if you have any queries, although the Coroner and his staff should keep you updated as well.