Thanks for the update. Much as I suspected. Pretty sure they weren't recorded on the main GRO index.
The legislation I've found is fairly recent - Scotland 1977, N. Ireland 2009, England/Wales 2013 and Ireland 2019. These mention maintaining a register.
Pre this legislation, I was wondering about (a) obtaining copies, if required, after the initial certificate was granted (b) updating the records where a person previously presumed dead was proved to be still alive. Paperwork has to exist somewhere!
All very intriguing.
Thanks again