Harold Wilkins (1916-?) was the last juvenile to be sentenced to the death penalty in the United Kingdom, in 1932 at the age of 16 though his sentence was later commuted due to his age. Wilkins was convicted of the "sexually related murder" of Ethel Corey.
The execution of those under the age of 16 had been banned by the Children Act 1908. The age was further raised to 18 in the year after Wilkins' conviction, under the Children and Young Persons Act 1933.
References
Harold Wilkins (murderer) Wikipedia(Text) CC BY-SA