I think there is a tendency to think more civil registrations of births and deaths are missing than is really the case. With some creativity and flexibility it is possible to find more of those 'missing' registrations in the index.
The obvious first possibility is the surname is mispelt. Ages can often vary sometimes considerably, but so can first names. I have a Henry whose death was registered as William, a Joseph whose death was registered as James and a James whose death was registered by his widow as John William Henry!
I also have instances of families, who appear consistently on censuses in one surname but register all their civil events in another, or people who change their names for whatever reason.
I research a one name study that goes into the low several thousands giving me a wide sample of people. In all I only hold 4 cases where I have a burial and no death registration, in 1838, 1840, 1843 and 1853. In three of those cases the death involved very young children. Only one burial was in a town.
I have done extensive searches of several of the large London cemetery records and some of the smaller non-conformist cemeteries in London so I have searched records not just where it is easier to do so in the more rural parish registers.
I do hold several cases where I have other records which indicate a person has died in an area, but I cannot find a death registration. One is a newspaper account of the person drowning in the Thames. The body if it was ever found further down the river, was registered as unknown. In the same quarter that man died in, 190 other people had their deaths registered as unknown. Over a hundred years later in the same quarter in a year in the 1970s 12 people had their deaths registered as unknown. That's a lot of unknowns over the years.
I also search civil registrations abroad and find the most unexpected people, who as far as I know never left the area they were born in, suddenly turning up dying in very unexpected places.
It is very rare however for me to have a person in my records who I know definitely died in this country, but I have yet to find a death registration for.
In the first few years of registration, when registrars were paid according to the number of births, marriages and deaths registered, some births were invented, and some records were ‘poached’ from adjacent registration districts. So though some unscrupulous people might have been interested in turning a blind eye to burying people without a death certificate, registrars would be very keen to seek them out, since it would be they who were out of pocket. Besides what reason would a person have for not obtaining a death certificate for the person who had died? If you were that poor the parish would bury the person and your loved one would get a Christian burial of sorts. To bury illegally robbed the dead person and the family of a Christian burial something that would be very important even to the poorest family. To pay someone to bury someone illegally would surely cost more than the death certificate and a pauper's grave.
Anyone in a church or cemetery who risked burying someone without a certificate risked being party to a death that had occurred not by natural causes. All burial places were required to keep records of their burials and in towns at least undertakers would also have to be involved in any illegal burying process and of course if you wanted a Christian burial the minister as well. Potentially alot of people to pay.
Unless you believe there was a flourishing trade in bodies entering cemeteries at dead of night and being buried by relatives or gravediggers who had had their palms crossed with silver, then the vast majority of burials are either in the index somewhere, not written into the central register because the return didn't make it to the central register, but the event is held by the local registrar (and the death was in fact registered), or accept in the end in a small percentage of cases that the death has slipped through the system and not been registered for whatever reason. You could also ask yourself how sure you really are that the person concerned actually was in this country when their death occurred.
Regards
Valda