The reason they are meant to be stunned before slitting the throat is so that they are not aware they are bleeding to death. Ipso facto, if the throat is slit without stunning, the animal is aware it is being slaughtered. Not nice.
Hitting any animal with a hammer, whether a rabbit or lamb, unless you have the veterinary knowledge to know that your single blow renders the animal immediately and painlessly unconscious, every time, seems to me to be far more cruel than alternatives.
I can't help feeling that, if people really are stuck with an injured lamb and too far from the vet, a farmer with a shotgun, or a knacker, then they should either have injectable drugs in their cupboard or, if this couldn't have been foreseen (eg, snowed in) then maybe drowning is kinder - although I am sure the animal would know it is being killed, it is at least, reputedly 'a nice way to go'.
But the whole conversation is distressing me - if I was situated where I may need to dispatch an animal myself I would feel obliged to have a gun and know how to use it for a quick safe certain kill. Or have meds in my cupboard and know how to use them ditto.