Prices are up this week so £50 for Shetland hoggets (Last year's Lambs) and £40 for old ewes should be achievable at market. Finished means well fleshed but not over fat.
The abattoirs are paying over £4 per kg for a whole carcase. Remember that Shetlands are smaller than the main stream cross breeds and that most of the costs after the abattoir door are per head so you will need to charge more per kg than a texel cross lamb.