without you weighing them first and seeing liveweight and condition scoring them its hard to tell!
Some lambs may take after the lleyn and some after the shetland,
my shetland cross lambs right now weigh from 40-46 kilo and just need a bit of finish to go, I sold a batch last week as stores to a friend as he needs some more to meet his customer demands
they generally kill out well due to being fine boned and crossed with a charollais, maybe 47-50%, so if they are fit at 45 kilo then they could have maybe 19-20 kilo of joints and meat etc in the box
No idea what lleyns would be like though, as they wont be as heavy as charlie lambs
Best thing to do is try and weigh them and feel for condition, good luck!