i had a old work colleague that was ex-abattoir man, i casually asked if they really did all the stunning b4 killing etc he said they didint bother half the time, jus killed em. dont know if that was true but u dont know what goes on behind closed doors.
They are stunned automatically. I can give a graphic description if needed. For the most part the pigs are respected, as the slaughter man said, if the pigs are stressed the meat is tough. There are cameras in all our local abattoirs. Most are brain dead before throat is cut, if not the slaughterer calls for the euthenasia callipers before finishing.
This is my experience of several abattoirs here. I am sure the UK which has high level of animal welfare rules would be better.