A Beltex
should be ok on Shetland ewes but definitely not first time lambers, especially for a first time owner. The whole point of meat from Primitives is that it has special, slow-grown properties, which set it apart from much larger, quick grown carcases. You do have to keep them on to 16 months, but they eat grass and hay through the winter, so are not expensive to keep, if you have the ground. If all you want is a large carcase, then crossing with a large tup is the way. For age, I would suggest getting a lamb is best, as he will be tasty, whereas an older tup could be a bit musty and greasy.
For getting your lambs slaughtered, Wishaw is too big and they can't really cope with just one or two. Shotts is better, being a small outfit, and they are happy to do small numbers. The guys there like the unusual sheep, rare breeds, and are friendly. You can take your lambs in the afternoon before, so they are starved when they go in, but they get water of course. They are also happy for you to come back a couple of hours later to collect the skins for tanning, if you let them know in advance. All slaughterhouses are horrible to go to, but then their job is a horrible one.
Yes, I know Larkhall, kind of. Well, I know about where it is
We are north of Biggar, so not so far away