I'm not sure if you are thinking of fattening wethers only, or keeping ewes and breeding. If you are thinking about milk, then I guess you are thinking of breeding females.
If it's unimproved grassland in Cumbria, and you want lambs, and want to sell fleeces, then I would say that 25 and no hay over winter won't work. You'd be likely to get ewes with insufficient milk, metabolic disorders, breaks in the fleece from times when they were physiologically stressed (fleece with a break is not saleable.)
Also, when counting your numbers, you have to think about your steady state. If you are going primitives, and fattening your own lambs, then most of your lambs will stay on until their second summer. So if you start with 25 ewes, the following winter you will have 25 ewes (less losses and culls) plus most of their 30-40 lambs from the first year growing on.
If you are wanting to sell fleeces, you'll need to keep the lambs on to their first shearing, which means that in your second spring you will have the ewes, their last year's lambs, now hoggs (adult sized, not yet clipped), and their this year's lambs.
If you plan on selling the lambs as stores, then you probably want to be using a commercial tup, or you will struggle to get more than £laughable a head for your store lambs. Shetland ewes are quite capable of having Texel x lambs - but it will take more out of her and she will need cake as well as hay over winter, and probably cake to help her when she lambs, especially if she has twins.
Primitives also need space when they lamb; they are not happy having to lamb in close proximity to other sheep. If you lamb indoors you will need a lambing shed; if you lamb outdoors then you need enough space so that the ewes can keep their new lambs safe from the rest of the flock, especially the giddy teenagers (the previous year's lambs) until they are used to these funny new arrivals.
I keep my fleece flock in a spot where I have the use of 25 acres of unimproved riverside meadow and pasture. At the moment down there I have 14 breeding ewes, 11 retained hoggs and 27 lambs from this year. The 10 acres of meadow was barely enough space and grass for them at lambing, so I let them use the pastures as well. (It was a very cold spring. It would have been a different story if we'd had warm weather.) The grass is finally growing away, and I have shut up the hay meadow. The cattle will be going down to the pasture soon, so the sheep will be barred into 6 acres of meadows. It'll be enough, I'm sure - but it wouldn't be sufficient for twice the number.
Frankly, on 6 acres of the type of ground you describe, I'd start with maybe 8-10 breeding ewes and their followers. Perhaps buy in some ewes with their ewe and wether lambs and some one-year old hoggs this year, so you have enough to eat the grass for your first year, hoggets to sell in the autumn, and a crop of fleeces and some meat to sell this time next year. By then your ewes will have had another lot of lambs and you are in your steady state.
And before all you southern and east coast sheepkeepers jump up, this fella is in Cumbria, like me. Our grass isn't like what you folks have. It's why our livestock sells so well as stores - you take creatures that have managed to get to weanling size in Cumbria, they'll grow on into monsters anywhere else! lol. Mind, we are at 450-500' here, very exposed (although the riverside land is lower and sheltered.) I have no personal experience of farming the land nearer the west coast.
