Shetland ewes are normally polled, so it would just be the tup, if not polled, and any unpolled tup lambs. If you get them ringed, they won't develop so much horn, and they'd be away before the horns were much of a problem.
You can get polled tups, but I'm not sure how easy they are to come by, nor to what extent his offspring would be polled.
Sounds like CRs might suit you better, though

There's also Polled Dorsets, of course. Fabulous meat, and able to breed at any time of year.