If the UK had retained the death penalty, then there would have been a potential of 58 executions of people whose convictions have subsequently been overturned, as they were victims of miscarriages of justice.
https://en.wikipedia.org/wiki/List_of_miscarriage_of_justice_cases#United_Kingdom
There are others presently in prison appealing their convictions. That list includes other countries with adversarial justice systems similar to ours, where there has also been a shocking number of miscarriages of justice regarding murders.
https://www.birmingham.ac.uk/research/perspective/death-penalty.aspx
In the USA "Since 1976, 1,348 people have been executed in the US, but in that time 136 people have been exonerated from death row on the grounds that they categorically could not have committed the crime for which they were sentenced to death. In other words, for every ten people on death row who are executed, at least one person on death row is innocent."
How barbaric is that? Anyone who wants the death penalty has to be able to say they would watch someone they know, whom they know did not commit a murder, die in front of them and still believe that was the right thing to do.