Boreray are small, yes, but the Soay is the smallest British breed.
I think Borerays are a similar size to North Ronaldsays. Yes you’d probably give them at least a year, or more depending on your ground, as for most of the northern short-tailed primitives.
Boreray are double-coated, very very hardy.
I know very little about Hill Radnors, except that the one fleece I’ve had was coarse and not very appealing to this handspinner.
Boreray can be very coarse too, and being double coated, takes more prep than some fleece. But it’s popular with handspinners because it’s so very rare, and a bit different. And there can be a bit of colour in it, so it’s interesting for dyeing. They’re not all super coarse and the undercoat can be quite soft, though very short.