Height above sea level is so variable in the uk , iv'e helped harvest cereal at 1100ft , spread fert and grass seed at 1500ft . The highest iv'e run sheep 2,500ft 36" rain deep snow blocking the roads digging sheep out -10 regularly but now 1000ft 80"+ rain snow and ice very rare and it is much much harder on the sheep . Mule ewe lambs spread out on a good hill would probably survive , but since you say you would sell them as hoggets , if for breeding they wouldn't compare with ones wintered and fed on good ground and if for killing I doubt they'd be fit enough to sell