Never underestimate the knackerman, most are top people and caring with a sh!t job. they do look when they collect and most will mention a problem if they spot it, I have a lot of respect for these people.
Sheep and lambs cost of collection: A ewe is a charge as one animal carcass, dead lambs during lambing etc, can be placed into a bin liner or similar and most will charge the bag at about the same cost as a single carcass. (Not sure if thats helpful but worth knowing I guess)
As to rules etc, damn good question? Worth looking into or even a call to your animal welfare, they don't mind answering quires, if you do and get a reply post it here, unless we have someone come along in the mean time.