In England, a real movement starts around the 1920s.

Prior to this, in the Victorian era and indeed further back, there had been individual objections to the notion of the death penalty, most famously by Charles Dickens, but while they did succeed in moving hanging away from being public events to taking place within prison walls, they had very little success in convincing the general populace.

By 1925, the National Council for the Abolition of the Death Penalty (NCADP) had been formed, in a response to the 1923 execution of Edith Thompson, a controversial case. Working together with the Howard League for Penal Reform, they gathered statistics to prove that the abolition of the death penalty had not negatively affected crime rates in other countries.

Then, in the 20s and 30s, there was reaction against the executions that took place during the First World War, mostly for "cowardice and desertion", and in 1930 and amendment was made to the Army Act of 1881, that stated that military executions were only to be undertaken as a result of treason.

It was only after the Second World War that the idea of abolition began to gain ground in Parliament, with the Labour government taking power, and in 1948 the Parliamentary Penal Reform Group got together a petition against the death penalty, with 187 MP's signatures attached. Eventually, a free vote was called in the House of Commons to suspend the death penalty, which passed with a very narrow margin, but was defeated in the House of Lords. Whilst another bill was proposed in 1955, it was once again defeated by the House of Lords.

It must be stated that very little of the early movement in Parliament against capital punishment was a matter of ethics. The discussions in Parliament between the 1930s and the 1950s were not so much about abolishing the death penalty entirely, but tended to be either about implementing certain "degrees" of murder so not every case was given the death sentence, implementing stricter regulations for mentally unstable patients, or finding better, more humane ways than long-drop hanging, the usual method in England at the time. The general feeling was that things could not continue as they were, not that the death penalty was a bad idea altogether.

The idea of capital punishment being unethical can be said to be largely influenced by three individual cases; Timothy Evans in 1950, Derek Bentley in 1953, and Ruth Ellis in 1955.

Timothy Evans' case can be read in more detail here, but essentially he was hanged for confessing to having killed his wife and daughter, despite there being no evidence, and later changing his statement to accuse his neighbour John Christie. His latter statement was ignored and he was hanged, but three years later John Christie was discovered to be a serial murderer, and the bodies of Timothy Evans' wife and child were found alongside several other of Christie's victims, and Christie himself confessed that he had indeed murdered Evans' wife. Despite several inquiries over the two decades afterwards, it was only in 2003 that his remaining kin were given "ex gratia payments as compensation for the miscarriage of justice in his trial."

Derek Bentley's case can be read in full here, but to summarise, Derek Bentley was a mentally handicapped young man who joined his friend Christopher Craig on a burglary. They were soon interrupted by police officers, and although Bentley was detained and made no attempt to fight or escape, Craig began to shoot at the officers chasing him, injuring one in the shoulder and killing another. At some point during the proceedings, Bentley is reported to have said "Let him have it, Chris," though whether or not he meant 'give him the gun' or 'shoot him' remains unclear, as does the validity of the claim he said it at all. Eventually, both him and Craig were taken to trial and charged with the murder of a police officer, and Bentley was sentenced to the death penalty.

Despite his mental issues, and although a large percentage of hanging sentences were reprieved, and 200 MPs signed a petition asking that he be reprieved, Derek Bentley hanged on the 28th of January 1953. On the 30th of July 1998 his sentence was deemed "unsafe" by the Lord Chief Justice.

Finally, Ruth Ellis, whose case can be read here. Ruth Ellis had been in a relationship with a man called David Blakely, but their relationship had a tendency to be angry and violent, and on occasion Ellis was seen with bruises from where she'd been physically abused. Eventually, Ellis began a relationship with another man, and despite Blakely's frequent affairs, he decided that due to this he no longer wanted to see her, and actively began avoiding her. On the 10th of April, 1955, Ellis took a loaded revolver, waited outside a pub in Hampstead where Blakely was drinking, and proceeded to shoot him six times. Unlike the previous cases, Ellis was definitely guilty of killing Blakely, and it was discovered she had no mental disorders or any insanity. The injustice that many of the public felt came from the previous behaviour of Blakely, and the judge's insistence that her crime should be considered murder, instead of manslaughter, which held a lighter punishment. Despite the judge later asking for a reprieve, Ellis was hanged on the 13th of July, 1955.

These three cases, all within five years, changed the public's opinion on capital punishment. All three of them inspired public marches and protests, as well as the reaction from a large number of MPs. Ruth Ellis's case sparked a public petition that received thousands of signatures.

In October 1964, Harold Wilson, a life-long supporter of abolition, won the general election for the Labour party. After a year of debating, the Murder (Abolition of the Death Penalty) Act of 1965 came into effect on 9 November 1965. This act, technically, suspended the death sentence for five years, leaving it up to the government in 1970 to decide whether or not to re-instate capital punishment, which they ultimately decided not to do. Though there were some practical points raised against re-instating it - the large number of reprieves, and the nonsensical idea of splitting murder into different categories for different punishments, the ultimate decision was made based on 'public decency' and 'an appeal to ethics'.

The final paragraph of the Home Secretary, James Callaghan's speech stated:

"‘Finally, my position is this. What we are discussing is the right of the State to exact the death of a citizen as a punishment for committing murder. That is an awesome power which is rightly denied to the private citizen. If it is to be given to the State then the onus of proof rests wholly and heavily on those who would bring it back as a punishment. I agree entirely with what was said by Lord Templewood, a former Home Secretary, that capital punishment lowered the moral standard of the whole community. That is my conviction, too. When society exacts this penalty it acts on the same level as the murderer himself.’" source

In 1965, the abolition of the death penalty became permanent, though capital punishment remained in place for high treason, piracy with violence, arson in the Royal Dockyards, and capital offences under military law. The Criminal Damage Act of 1971 abolished the death penalty for arson in the Royal Dockyards, as well, and then the Crime and Disorder Act 1998, s 361 and the Human Rights Act 1998, s 21(5) abolished the death penalty entirely.

Although there were 13 attempts by the Conservatives between 1965 and 1994 to re-instate capital punishment for various crimes, they have, so far, all been rejected in the House of Commons.

In summary, the movement towards finding the death penalty unethical started, largely, due to the miscarriages of justice during the First World War, gained ground due to a combination of solid statistics proving it was not necessary and the politicians' belief that grading murder was an inefficient system, and was finally a majority belief due to three trials where people may have been wrongfully hanged.

Sources:

http://www.deathpenaltyproject.org/wp-content/uploads/2015/11/DPP-50-Years-on-pp1-68-1.pdf

http://www.capitalpunishmentuk.org/contents.html