It's hard because we don't know your climate. Clearly, from the posts in this thread, there's a heck of a difference between moorland in North Yorkshire and in southwest Northumberland, which are nearly next door to each other, but N Yorks is further south and east and enjoys significantly easier conditions than southwest Northumberland.
Best thing would be to ask local sheep farmers.
. Also about which lambs to buy, and from whom/where - some producers will produce hardier sheep than others.