Your legal duty is to satisfy the state that the cause of death has been reported by a ‘qualified person’ within 5 days: excellent info here. A ‘qualified person’ is defined here at 16 & 17 (a funeral director is not allowed to do this; an executor who is not a relative cannot do this either, unless there is no relative and the executor was present at the death.)
The purpose of registration is to ensure that the death has been certified and recorded; foul play has been ruled out; and accurate health and disease information has been recorded.
So: the death must be certified by a doctor who provided care during the last illness and who saw the person who has died within 14 days of death (28 days in Northern Ireland) or after death.
There is no legal definition of death in the UK
The death must be notified by a qualified person to the registrar within 5 days and registered within 14 days:
The registrar will issue:
a Death Certificate — proof that the death has been registered
a Certificate for Burial and Cremation, also known as the green form, giving the go-ahead for the person’s body to be buried or for you to apply for cremation. A green form will not be issued if a coroner needs to issue an alternative document for a burial or cremation. If the person who died was reported to the coroner, find out what happens next here.
a Certificate of Registration of Death (form BD8) for use in dealing with the dead person’s state pension or other benefits.
If you want to bury the person who has died you can now go ahead – unless the death happened in England or Wales and you wish to take the body for burial or cremation to another country, even Scotland, Northern Ireland, the Isle of Man or the Channel Islands. You’ll need to have the permission of the coroner. You apply for this by filling in Form 104 (Form of Notice to a Coroner of Intention to Remove a Body out of England). The registrar should be able to give you that form – as mentioned at the bottom of “Part B” on the green form which the registrar will give you.
If the death happened abroad, see here.
Failure to register a death is a summary offence punishable by a fine.