Anyone who thinks all animals are stunned before they are killed is living in cloud cuckoo land. You only have to google it and you will find the most horrific videos and pictures. I shan't post any I'll leave it to your imagination. This list from the Telegraph on which supermarkets sell which Halal products is 3 years old but interesting nonetheless. Oh and M&S's West Country lamb used to be non Halal but last time I checked with them they wouldn't confirm it was still so.
Where is halal meat sold?
SUPERMARKET OR RESTAURANT
IS HALAL MEAT SOLD?
ARE PRODUCTS LABELLED AS HALAL?
Waitrose Yes. Some lamb is halal. All other meat is non-halal No halal labelling on New Zealand lamb
Marks and Spencer Yes. All New Zealand lamb is halal. Chicken, beef and British lamb is non-halal No halal labelling on New Zealand lamb
Tesco Yes. Most New Zealand lamb is halal and some other meats use halal techniques. Some halal-only counters At halal counters and kosher ranges
Morrisons Yes. Some branded halal chicken, lamb and beef. New Zealand lamb is halal No halal labelling on New Zealand lamb. Halal ranges show slaughter method
Sainsbury's Yes. A range of halal and kosher products Halal and kosher ranges are labelled
Fortum and Mason No N/A
Asda Yes. A range of halal and kosher products Only branded halal and kosher meals
Pizza Express Yes Not on menus, but on website
Zizzi Yes No
Ask Yes