A typical 'butcher's lamb' is around 25-27kgs deadweight. Most of us on here aren't sending animals that chunky, so I guess most will be less than that.
I don't know if anyone's ever measured how much of the 'deadweight' disappears (fat, bone, gristle, etc, not to mention some weight loss by drying out during hanging) between slaughter and butchering?
My recent Jersey heifer was 192kgs on the hook, approx. 116kgs butchered (no bones in meat.)
I sent 4 fully adult (three summer) Castlemilk Moorit wethers, weighed about 140kgs between them on the hoof, got 55kgs meat back - that's approx. 14kgs per animal. Adult Shetlands would be similar, I would think.
I was going to say that no, I don't think you can sell that for £70 a half - but that people will pay for it and come back for more, it's tremendous meat. However, at £7-£10/kg, that is £50-£70 a half, so maybe you can!