Information sourced from UK Government.
Halal and kosher slaughter methods are legal in the United Kingdom. Around 9% of birds are killed using these methods. Most birds killed using non-stun methods are chickens specifically raised for their flesh, rather than spent egg laying hens.
Halal is a term used to describe foods prepared and processed in a way that is permissible under Islamic law. The process of killing animals, known as dhabiha, involves cutting the throat to sever the jugular vein, carotid artery and windpipe. Animals are blessed during this process, although it has no effect on welfare.
Kosher is a term used to describe a set of Jewish dietary requirements, with one of those requirements being the method in which an animal was killed. Chickens are typically hung upside down and their throats are cut using a sharp blade. This method is known as Schechita. A study of birds slaughtered in a Kosher manner found that around 60% showed response to stimuli up to 5 seconds after slaughter. At 12 to 15 seconds the birds showed signs of involuntary muscle contractions.
While halal and kosher slaughter methods cause an immense amount of suffering it's important to remember that the same is true for stun slaughter methods such as using a gas chamber.