Blackies or Swales, put a Leicester onto them to produce mule lambs. They'll sell well, it's an accepted set-up in the hills.
Shetlands can be bucket trained, but small and grow slowly, I'm about to try a crossing tup on mine but even so, they won't be the size of commercial sheep. You'd have to sell purebreds at specialist sales, not the general mart as has been said.
Cheviots are a good solid hill sheep - haven't kept them, a few folk round here do, I like the look of them. I suspect they need better grazing that the ubiquitous Swales who apparently can survive on mist and moss.
I used to keep Rough Fells. Huge mountain sheep, very friendly, tups away at 6 months to the butchers, make a cracking mule or cross. Not numerically a rare breed, as there am some big flocks of them locally, but included as very geographically constrained. I love them to bits and would still have them if I had better sheep handling facilities (they became too big for me to manage in wooden hurdle pens, turning by hand).