All non-halal, non-kosher meat is stunned and then exsanguinated - major blood vessels severed and then raised up so that the blood drains out. If you don't do that, the blood sits in the meat and spoils it very quickly, makes it taste awful and metallic I believe.
The exact process will vary from species to species - pigs for example are electrically stunned using a pair of tongs that go either side of the head, passing the current through the brain - then the hind leg/s are shackled to an overhead chain pulley system - throat cut, then the animal is moved on a runner system, still suspended, to the butchering areas.
Cattle and sheep are stunned with a penetrating captive bolt - so a bolt comes out of the gun, penetrates the brain which renders the cow unconcious - then the same as above though some abbatoirs use a crush that tilts, then the chain shackle lift procedure.
Chickens - gassing - they stay in their transport crates and are passed on a conveyor belt through a controlled atmosphere, basically a gassing room, and then are quickly processed as the crates come out the other side before they wake up.
Electric waterbath - some places still use this though it is much more stressful - each animal is removed from the transport crate, strung up by a leg on a conveyor track system - that moves them head down into a waterbath that passes an electrical current through the brain - then immediately out of the waterbath the throat is cut so they bleed out. However, they can lift their heads up and swerve the water bath, and if the cut isn't done fast enough they can regain conciousness before it happens.
With most halal meat produced in the UK, all of the above happens but... the killing is done by a muslim using the correct equipment and prayers are said (usually via a PA system on a loop) as thats done. Just a tiny amount of halal slaughter happens without the pre-stunning, but exsanguination via a very sharp knife is very quick, and not especially painful if its done properly. I don't think its good, but I also don't think it adds a significantly large amount of stress compared to the rest of the process!