Question Regarding Gap in UK General Register Office Death Records

The UK's General Register's Office has been a great source of finding birth and death records during my geneaology research, however there is a gap in the death records between 1958–1983. I do not believe it has anything to do with privacy laws (though I could be wrong) because I was able to get a great-aunt's death record who passed away in 1990.

My questions are as follows: (1) why does this gap exist and (2) are there other ways to access an ancestor's death record who passed away during this time period? TIA.