FYI, Longtown Mart's average liveweight price yesterday was around 183 ppk. So if your lamb weighs 45kg on his hooves at home, £80 would be a very good price for him/her in the ring. Pet/orphan lambs usually get 10-15%, or even more, less than a good average commercially-reared lamb, so £65-£70 would probably be a more realistic expectation. (Note, all lambs will lose a kilo or two between the scales at home and being weighed at the mart.)
Our butcher charges £20-£25 for slaughter and butchering. All cuts are wrapped suitably for freezing right away and are labelled with cut and weight.
So your £100 per lamb is probably just about right, Mandy.
BTW, for pet/orphan lambs you would be pleased to get 45% deadweight, ie, 45kg at home returning just over 20kg meat. At £100 per whole lamb, that'd be £5/kg overall.