Chemical weapons were widely used by the United Kingdom in World War I, and while the use of chemical weapons was suggested by Churchill and others postwar in Mesopotamia and in World War II, it appears that they were not actually used, although some historians disagree. While the UK was a signatory of the Hague Conventions (1899 and 1907) which outlawed the use of poison gas shells, the conventions omitted mention of deployment from cylinders, probably because that had not been considered.
During the First World War, in retaliation to the use of chlorine by Germany against British troops from April 1915 onwards, British forces deployed chlorine themselves for the first time during the Battle of Loos on 25 September 1915. By the end of the war, poison gas use had become widespread on both sides and by 1918 a quarter of artillery shells were filled with gas and Britain had produced around 25,400 tons of toxic chemicals.
Britain used a range of poison gases, originally chlorine and later phosgene, diphosgene and mustard gas. They also used relatively small amounts of the irritant gases chloromethyl chloroformate, chloropicrin, bromacetone and ethyl iodoacetate. Gases were frequently mixed, for example white star was the name given to a mixture of equal volumes of chlorine and phosgene, the chlorine helping to spread the denser but more toxic phosgene. Despite the technical developments, chemical weapons suffered from diminishing effectiveness as the war progressed because of the protective equipment and training which the use engendered on both sides.
Mustard gas was first used effectively in World War I by the German army against British and Canadian soldiers near Ypres, Belgium, in 1917 and later also against the French Second Army. The name Yprite comes from its usage by the German army near the town of Ypres. The Allies did not use mustard gas until November 1917 at Cambrai, France, after the armies had captured a stockpile of German mustard-gas shells. It took the British more than a year to develop their own mustard gas weapon, with production of the chemicals centred on Avonmouth Docks. (The only option available to the British was the Despretz–Niemann–Guthrie process). This was used first in September 1918 during the breaking of the Hindenburg Line with the Hundred Days' Offensive.
The use of chemical weapons in warfare during the Great War was in violation of the 1899 Hague Declaration Concerning Asphyxiating Gases and the 1907 Hague Convention on Land Warfare, which explicitly forbade the use of "poison or poisoned weapons" in warfare.
To maintain a stockpile of Adamsite, Ministry of Munitions established Sutton Oak Chemical Defence Research Establishment (CDRE) in 1919. The plant was able to manufacture up to 20 tons of mustard gas per week in the late 1920s. 
After the war, the Royal Air Force dropped diphenylchloroarsine, an irritant agent designed to cause uncontrollable coughing, on Bolshevik troops in 1919, and Winston Churchill, secretary of state for war and air, suggested that the RAF use various chemical agents in Iraq in 1920 during a major revolt there. However, historians are divided as to whether or not gas was actually used.
Britain signed and ratified the Geneva Protocol in 1930, which banned the use of toxic gases and bacteria in war but not the development and production of such weapons. Britain carried out extensive testing of chemical weapons from the early 1930s onwards. In the Rawalpindi experiments, hundreds of Indian soldiers were exposed to mustard gas in an attempt to determine the appropriate dosage to use on battlefields. Many of the subjects suffered severe burns from their exposure to the gas.
In the late 1930s the Chamberlain Government planned that the United Kingdom should be in a position at the beginning of any war to retaliate in kind if the Germans, as expected, used mustard gas and phosgene. to help repel a German invasion in 1940–1941, and had there been an invasion may have also deployed it against German cities. General Brooke, in command of British anti-invasion preparations of World War II said that he "...had every intention of using sprayed mustard gas on the beaches" in an annotation in his diary. The British manufactured mustard, chlorine, lewisite, phosgene and Paris Green and stored it at airfields and depots for use on the beaches.
In the late 1930s the Chamberlain Government planned that the United Kingdom should be in a position at the beginning of any war to retaliate in kind if the Germans, as expected, used mustard gas. In April/June 1939 the Alyn Valley in Rhydymwyn was surveyed by the Department of Industrial Planning on behalf of the Ministry of Supply and Imperial Chemical Industries (ICI) who were tasked with managing this programme. This resulted in M. S. Factory, Valley as United Kingdoms main chemical weapons plant.
To fast be able to retaliate if Nazi Gearmany used chemical weapons, a number of Forward Filling Depots was built so the mustard gas stockpile was dispersed and ready to use.
The mustard gas stockpile was enlarged in 1940-1941 to help repel a possible Nazi German invasion German invasion in 1940–1941, and had there been an invasion may have also deployed it against German cities. General Brooke, in command of British anti-invasion preparations of World War II said that he "...had every intention of using sprayed mustard gas on the beaches" in an annotation in his diary. The British manufactured mustard, chlorine, lewisite, phosgene and Paris Green and stored it at airfields and depots for use on the beaches.
Winston Churchill issued a memorandum advocating a chemical strike on German cities using poison gas and possibly anthrax. Although the idea was rejected, it has provoked debate. In July 1944, fearing that rocket attacks on London would get even worse and that he only use it if it was "life or death for us" or would "shorten the war by a year", Churchill wrote a secret memorandum asking his military chiefs to "think very seriously over this question of using poison gas." He said "it is absurd to consider morality on this topic when everybody used it in the last war without a word of complaint..." and that:
I should be prepared to do anything [Churchill's emphasis] that would hit the enemy in a murderous place. I may certainly have to ask you to support me in using poison gas. We could drench the cities of the Ruhr and many other cities in Germany ..., We could stop all work at the flying bombs starting points.... and if we do it, let us do it one hundred per cent.— Winston Churchill, 'Most Secret' PRIME MINISTER'S PERSONAL MINUTE to the Chiefs of Staff, 6 July 1944
The Joint Planning Staff (JPS), however, advised against the use of gas because it would inevitably provoke Germany to retaliate with gas. They argued that this would be to the Allies' disadvantage in France both for military reasons and because it might "seriously impair our relations with the civilian population when it became generally known that chemical warfare was first employed by us." The JPS had similar concerns about public morale in Britain, fearing that people might become resentful if they felt a gas war could have been avoided. The Chiefs of Staff also warned that the Nazis would have no particular "difficulty in holding down the cowed German population, if they were subjected to gas attack," whereas the British population "are in no such inarticulate condition." Also that the German might also use Allied prisoners as workers in contaminated areas causing "great public concern".
Churchill responded to this advice by saying:
I am not at all convinced by this negative report. But clearly I cannot make head against the parsons and the warriors at the same time. The matter should be kept under review and brought up again when things get worse.
At the same time, the JPS examined the case of using anthrax bioweapons against six large German cities but only ruled this out on the grounds that the anthrax bombs were not yet available. A large batch of aerial bombs were ordered, but by the time the US factory was ready to produce them, they were deemed unnecessary since the war in Europe was almost over.
Novelist Robert Harris and broadcaster Jeremy Paxman argue that as soon as another weapon of mass destruction – the atomic bomb – became available, and offered a chance to shorten the war, the Americans used it. "Why, from an ethical or political point of view, should germ warfare have been regarded any differently? [by British]."
As the end of the war was sufficiently in sight, British poison gas production was terminated following a request from the Chiefs of Staff in February 1945.
From 1939 to 1989 experiments on chemical weapons including nerve agents and countermeasures were carried out at the Porton Down research establishment. Although volunteers were used, many ex-servicemen complained about suffering long term illnesses after taking part in the tests. It was alleged that before volunteering they were not provided with adequate information about the experiments and the risk, in breach of the Nuremberg Code of 1947. This became the subject of a lengthy police investigation called Operation Antler.
From 1950, a Chemical Defence Establishment was established as CDE Nancekuke for small-scale chemical agent production. A pilot production facility for Sarin was built, which produced about 20 tons of the nerve agent from 1954 until 1956. A full-scale production plant was planned, but with the 1956 decision to end UK's offensive chemical weapons programme it was never built. Nancekuke was mothballed, but was maintained through the 1960s and 1970s in a state whereby production of chemical weapons could easily re-commence if required.
In the early 1980s the government took the view that the lack of a European chemical weapons retaliatory capability was a "major gap in NATO's armoury". However the political difficulties of addressing this prevented any redevelopment of a British chemical weapons capability.
An inquest was opened on 5 May 2004 into the death on 6 May 1953 of a serviceman, Ronald Maddison, during an experiment using sarin. His death had earlier been found by a private MoD inquest to have been as a result of "misadventure" but this was quashed by the High Court in 2002. The 2004 hearing closed on 15 November, after a jury found that the cause of Maddison's death was "application of a nerve agent in a non-therapeutic experiment".
I want the matter studied in cold blood by sensible people