For commercial sheep outside we use one small bale per 30-40 sheep per day as a guide. Double that for indoors, or outside if there's snow on the ground or the ground is frozen, or if it gets so wet and muddy there's no real value in the grass. Mind, we do also feed ewe rolls indoors, or if there's snow on the ground or the ground is frozen for more than a few days. We're in north Cumbria, on severely disadvantaged hill ground.
For 10 Dorsets, you'll probably want to give about 1/3 bale a day, I'd think, more in extreme conditions or if you find they're losing too much condition.
Our small bales are well-packed, about 40 to the tonne, of our own hay. You may need to feed more if your bales are light, or less if your hay is made from better grass than ours.
Edited to add that we generally start feeding hay about Christmas time. Earlier if the weather and ground conditions dictate.
As to straw, you only need straw if they'll be indoors, although if the ground is covered with snow or is frozen solid, they will lie on straw if you put some out. (We usually feed the hay spread out on the ground, and they lie on the spoil.). Although maybe you were thinking to feed straw rather than hay?