Years ago I had a friend who was an Environmental Health Officer. He said that Kosher slaughter was very very quick and sure; there is a rule that if any matter adheres to the knife then the whole carcase isn't kosher and must be discarded - so the knives are very very sharp and the deed is very very quick.
At that time, he felt that Halal slaughter was less humane, in that knives were not always so sharp and therefore things were not so swift. Since then, there has been a whole lot more legislation, and all abbatoirs have to pay for a qualified veterinary officer to oversee the handling of livestock.
All abbatoirs, whether Halal, Kosher or mainstream, are subject to the same policing and welfare standards. Therefore we should, these days, be able to be confident that any animal slaughtered
in these Isles will be handled and slaughtered humanely.
Of more concern is the number of ewes and lambs which are sold through our marts and leave the country on boats bound for who knows where and what level - or lack of - humanity in the slaughter that is their eventual end.