The Independent,
"Yet now the RSPCA says that, when compared to methods that involve stunning the animal beforehand, it can cause unnecessary suffering, pain and distress."
http://www.independent.co.uk/life-style/food-and-drink/what-is-halal-meat-the-big-questions-about-religious-slaughter-answered-9331519.html
I believe the RSPCA.

The Guardian,
"The president-elect of the British Veterinary Association (BVA) John Blackwell said he wanted to discuss the (slaughter) issue with Jewish and Muslim groups in order to find a compromise that puts more emphasis on the welfare of the animal.
"I don't think an outright ban is a long way off, there is enough of a view that this practice is inhumane and causes suffering at the time of death," said Blackwell."
https://www.theguardian.com/world/2014/mar/06/reform-of-kosher-and-halal-slaughter-practices
I believe him.