I second that the knackermen can also kill your animal and they do it quickly and cleanly, with no stress to the sheep. We have used this service a few times and it means a dying sheep doesn't get put through the stress of being moved around or leaving the familiar behind. It also means you don't have an extra callout fee for the vet to pay.
Our company, Grayshill, are very good about the speed with which they attend. If you phone in the morning they will usually come that day, or the following day at the latest.
If we have found a sheep dead we put it in a wheelbarrow outside the gate, covered with a bag for the sake of the public, and they remove it with no fuss.
If we need one killed we put it in a small pen as near to the road as possible, so the knackerman isn't traipsing bugs and disease across our land, then Mr F goes with the man to keep the sheep calm while he does the deed.
This company incinerates the carcases and they are not eaten by anything.
Very sensible to have this sorted out in advance, rather than waiting until it happens to you and you don't know how to deal with it.