CG76723 | Wasting assets: copyright
From HM Revenue & Customs · Capital Gains Manual
The copyright in any work runs for a specified period. Currently, it runs for 70 years after the end of the calendar year in which
the author, etc, dies
or
the work is first published or produced.
Thus, if copyright is acquired by a person 20 years or more after the end of the year in which the author died, it is then a wasting asset.
Before extension to 70 years copyright ran for 50 years and was a wasting asset from the end of the year in which the author died or the work was first published or produced.
CG68250 tells you more about copyright.