I'm not sure why you'd be considering mashams for fat lambs?
Mules and Mashams are produced as breeding females; the male lambs do fatten but they will never be the top sellers.
Nothing out of a Swaledale or Blackie is going to be an excellent fat lamb.

And mostly they won't finish in the first year, or certainly not the majority of them.
Cheviots are excellent sheep, and the lambs sell very well as stores and when fat. They take longer to finish, so you wouldn't get them fat in the first year - but that's why they sell well as stores.

We are on SDA ground, a hill farm in north Cumbria, and Texel type commercial ewes do well here. So you don't necessarily need to have a traditional hill breed on hill ground, and if your objective is a fat lamb then a heftier ewe would give you a better lamb.
Another option would be Mules, either North of England or Scotch, put to a Texel. Better conformation than what comes out of a Swale or Blackie, although still not a top fat lamb.
You could also consider Lleyn, Romney, Jacob, Black Welsh Mountain, Llanwenog, to name but a few. They're all pretty hardy and give a reasonable fat lamb when put to a terminal sire.
Would you be wanting to get your fat lambs away in the same year? Where/how would you be selling them?