They had these aircraft before 1918.

I've read accounts of the Brits experimenting with bomb throwers in 1914 (and they were very pleased with the results).  I have no idea how the Brits delivered whatever chemical weapons they may have used, but it doesn't seem like planes were necessary to accomplish the task.    

Winston Churchill was actually behind the exercise, which has led plenty of people to compare Saddam to Churchill...

While you can doubt Wikipedia, it is no different to any other publication at the end of the day - history is rewritten again and again - and biased reporting and propaganda are just as likely to appear in the Washington Times or on Fox News (or Prensa Latina or the World Socialist website if you want to go the other way) as they are on Wikipedia (which at least tries to be objective and has accompanying debate and citations with each article).

Here's some history from a Kurdish web site, who presumably don't have any ideological axe to grind with the British nowadays.

The Kingdom of Kurdistan did not last long, thanks to the British Royal Air Force acting on behalf of a puppet government in Baghdad. The British were not much kinder to the Kurds. It is wrongly preserved that the first regime that used poison gases against Kurds was Saddam Hussein's government. This is wrong. British were the first regime to gas Kurds in South Kurdistan.

In this book, 'Deterring Democracy', Noam Chomsky describes British rule in South Kurdistan as follows: [1]

As Secretary of State at the War Office in 1919, Churchill was approached by the RAF Middle East Command for permission to use chemical weapons 'against recalcitrant Arabs as experiment.' Churchill authorized the experiment, dismissing objections:

I do not understand this squeamishness about the use of gas. I am strongly in favour of using poisoned gas against uncivilized tribes. It is not necessary to use only the most deadly gases; gases can be used which cause great inconvenience and would spread a lively terror and yet would leave no serious permanent effects on most of those affected.

Churchill added: 'we cannot in any circumstances acquiesce in the non utilization of any weapons which are available to procure a speedy termination of the disorder which prevails on the frontier.' Chemical weapons were merely 'the application of Western science to modern warfare.'

Churchill was in favour of using air power and poison gas against 'uncivilized tribes' and 'recalcitrant Arabs' i.e. Kurds and Afghans [2]. Not surprisingly, in the 1990s, William Waldegrave, who was in charge of Prime Minister John Major's 'open government' initiative, ordered the removal from the Public Record Office of 'files detailing how British troops had used poison gas against Iraqi dissidents including Kurds in 1919 [2].

In this way, a people who wished to run their own affairs were oppressed to the limit of genocide. Their King was undermined by the mighty British forces and an 'imported' King from totally different culture was forced upon them.

The history has been written with the blood of the oppressed by the oppressing people. When the writings are still wet, the concept of `civilised and uncivilised' people emerges. The more oppressed people bleed, the larger the history books get. The history fails to report commensurately the suffering of the oppressed. In this way, we build, what we term, 'civilisation'.

You can find plenty more on the topic at Google.