Most abattoirs will cut into basic joints/chops but if you want anything more than basic joints it's best to find a butcher that is willing to pick up the carcass, take it back to their butchers and cut it for you.
Cost of kill and cut is £25, £12 to kill and £13 to cut and I think this is about the going rate. Anything like mincing, vacuum packing, de- boning joints would incur extra charges.
IF you want to know the realistic cost of producing lamb for your own table, you need to take into account care of the ewes throughout the year, so worm/fluke drench, flystrike preventative (crovect or clik), hay during the winter months if short of grass, not to mention vet bills if things go wrong.
Are these your first sheep? It might have been a wiser idea to buy some store lambs, run them on through winter and then kill in the spring just so you can get to grips with sheep before adding in breeding. The other thing to keep in mind is what you'll do with a ram if you buy one, as he will need to be separated from the ewes for the majority of the year.