Just in case the terminology needs some explanation:
Prime or primestock is animals that are ready to slaughter. Most animals sold in the prime (also called fat) ring will go straight to an abattoir - although some buyers will pick up animals that would benefit from a bit of finishing and take them home and feed them up a bit.
Generally these animals are priced per kg liveweight. The batch is weighed on the way into the ring, and bidding is pence per kilo. Buyers may feel the animals either in the ring or on the way to the ring - they’ll know what conformation they’ll hit from a quick squeeze or two.
The store ring is animals that are generally not ready for slaughter (although sometimes there are some in there which are in fact just right, and may be bought and shipped direct to abattoir.). Also sometimes called feeding sheep. (Need more feeding before they’re ready for slaughter.).
Generally store animals are priced per head. They’re not weighed and bidding is pounds and pence per head. Again, buyers may feel the sheep to judge how much feeding they’ll need and what their underlying conformation is.
You would think that a pen of five fat lambs in the prime ring would always fetch more than a pen of five similar lambs, not yet ready for slaughter, in the store ring - but it is not always so. A buyer of store lambs is effectively buying futures - the price those sheep will fetch later in the year, when they’re ready. Sometimes you’ll see small, skinny lambs fetch more in the store ring than well-finished lambs in fat ring at the same market on the same day. All sorts of factors play a part here.
What the commercial farmers in Shetland do is cross the Shetland to a Cheviot. The ewe lambs are retained for breeding and are put to a Suffolk for fat lambs; the wethers from the first cross are ideal overwintering store lambs for a dairy farmer and should sell well at the end of the summer. The first cross ewe lambs also sell well as breeding sheep.
Cheviot lambs are born small and grow from late autumn onwards, making corking hoggs to sell fat after New Year, before the spring lambs are ready.
A Shetland ewe will take a Suffolk tup and produce a couple or three crops of good lambs for you, but the heavier birth weight lambs will take their toll on her belly muscles.
It might be worth having a chat with the auctioneers at your local mart to see what cross they think would suit you and their customers.