not very. I've looked into worldwide capital punishment stuff before and this duke article seems to fit with what i've seen:

Despite public opinion polls showing that 60 to 70 percent of the electorate favored retention, Parliament imposed a five-year ban on capital punishment [in 1964 before the ban for non-treason crimes 5 years later]

and in 1995 reinstatement of capital punishment in the UK was supported by up to 76 percent of the population

essentially abolition of capital punishment in the 20th century has been an elite driven phenomena helped out by changes in international norms and codified in international agreements rather than pushed initially my mass popular appeal.