Fat lamb prices have been falling since the Olympics ended. Top commercial lambs are struggling to fetch more than £75 in the ring at the moment, and deadweight sales are similar.
£25 for slaughter and butchering seems to be about the going rate, so you'll be doing fine if you can sell your lambs at £60 / half lamb.
However, I do agree that home-reared lamb is superior to much of what you find at the supermarket, and if you look at prices on the butchers' shelves, you could certainly get away with £8/kg, or more for a well-finished lamb, I would have thought.