Micron count is just one factor, and is not the be-all and end-all.
When we had Deborah Robson (internationally recognised fleece expert, co-author of the seminal work The Fleece and Fiber Sourcebook, and previous editor of Interweave magazine) to give us workshops on our own local sheep, she made thre point that possum, the finest fibre of all at 5 microns, is so short it is hard to make yarn which isn’t intensely itchy, because of all the ends of fibre sticking out! And I’ve experienced blending cashmere (another fine but short fibre) into a mix to soften it, but finding the same - soft to stroke but itchy to wear.
Another eye-opener from our workshop with Deb. I was in the North of England at the time, and we studied five of the six English Northern blackfaced hill and mountain sheep first, plus the Hebridean, then the Blue-faced Leicester, the North of England Mule (blackfaced hill ewe x BFL tup), the local commericlal sheep type (known as the Soft Fell, being a mix of Mule, Texel and a few other ingredients) and finally the Castlemilk Moorit. (I must have forgotten two, I’m sure we studied 12 in all!)
The hill sheep we studied were Rough Fell, Lonk, Hexhamshire Blackface, Swaledale and Derbyshire Gritstone. (I hadn’t managed to source a Dalesbred Fleece.). All but the last are double-coated (as was the Heb) so you could separate the coarser, longer outer fibres from the shorter, very much softer inner layer. Between us we tried pretty much all combinations - outer only, inner only, and mixed, and experimented with combing, carding, spinning from the lock, from the fold and from the combs, and spinning woollen, worsted and all points in between. I was amazed at the softness of the woollen-spun inner coat of the Swaledale, but the biggest eye-opener for me was a sample of Hexhamshire Blackface, both layers together, softly woollen spun. It was such a gorgeous soft-feeling yarn, I popped it in my bra for the ultimate prickle test. Late in the afternoon I remembered I’d put it in there and actually wondered if it had fallen out, because I simply wasn’t aware of it at all. (And I am a person who is a bit sentive to prickly wool.)
Why is lambswool generally softer? Because one end of the fibre isn’t cut, perhaps? Being the first clip, the outer end is the tip of the fibre the sheep was born with. All subsequent fleeces have two cut ends. (Unless the sheep are roo-ed of course.).
In my own view, the sharpness of the cutting blades, and the skill of the shearer (which includes maintaining the cutting equipment during shearing) have a huge impact on the fibre. A clean sharp cut will not prickle nearly so much (if at all) as a cut made with blunted blades, in a set of clippers in need of being cleaned of debris and lanolin. In the extreme, the latter is less of a cut than a tear, and will leave a ragged end with a twistiness that makes it escape even the tightest of worsted spins and prickle the wearer, however fine the fibre to which it belongs.
And don’t get me started on pilling, and the lamentable quality of some big brand British wools...
So the upshot of all of that is to say that many, many factors affect the prickle factor. If you embark upon this venture because you like sheep and will enjoy learning about what affects the yarn that is made, you’ll have enormous fun and might end up with yarn that suits your need. (Though possibly not every year ;p). If you really need to be producing repeatable yarn that gives you the soft but strong yarn you need, I think you might struggle - not least because only one Mill in the UK can produce a truly combed preparation, and that’s John Arbon in Devon - and he only does runs in the tonnes. (And the difference is evidenced by the fact that any spinner who’s used his tops goes back to him again and again and again...
).
I think I’d start by asking the Mills you might use for samples of yarns they’ve produced that they think might meet your need. Then you can at least start from a position where all you need to get right is the fleece; you know the Mill can do the job with the right input. And the Mills will be able to advise on sheep breed too, of course.
My bet is that, if you do get to the point of having sheep, they’ll be carefully-selected Wensleydale or Teeswater, or a cross of those with Blue-faced Leicester, or a Leicester Longwool. (I’m actually daydreaming about crossing a Leicester Longwool with a Doulton Flock Border Leicester now...
).
Karen of Griffiths Mill made a formal frock (for the RBST anniversary dinner) from Wensleydale, so I’d start there, I think. And it’s always worth talking to the Natural Fibre Company; they’ve got enormous experience.