Castlemilks being horned, I really wouldn't worry about them competing at the trough. Mine give our 80kg+ Texel ewes a run for their money!
I have Manxes and Castlemilks. Both are horned, of course. My Manxes so far seem to be super crossing mothers - hybrid lambs growing big and strong. But I haven't tried a full-on commercial tup on them yet; I used a Shetland x commercial (1/4 Charollais, 1/8 Beltex) tup on them this time, and the lambs are looking good - at this point they're as big as the ones out of Dutch Texel ewes to the same tup.
Oh, and my Manx fleeces are
divine.

If you want to help a rare breed, then neither Southdown nor Ryeland are in the 'at risk' categories at the moment - they're both 'Success Stories', which is great, eh?

Teeswater and Wensleydale are both still at risk (in fact, Teeswater is one category higher, 'Endangered', like Castlemilks), lovely for spinning, can have good fat lambs either purebred or crossed.
I span the Norfolk Horn for the RBST Rare Breeds Blanket last year, and have wanted to get my hands on some more fibre like that ever since. Norfolk x Southdown is what created the Suffolk, so a Norfolk should be able to birth and rear a fat lamb, I would have thought. Purebred lambs are really sweet

Oxford Down would perform similarly to Southdown, I'd have thought, and is still at risk.
Border Leicester is a big sheep, so no problem with producing a fat lamb, and stupendous fleece.

They're in the minority category, so it would be good to have some out of their home area. (Can I have a fleece if you get them?

)
Equally, Dorset Horn and Dorset Down both have lovely fleece and produce good fat lambs, and are in the Minority category. They probably meet your spec as well or better than any, I think.
I love choosing! I like helping other people choose
nearly as much as choosing my own!

The only other factors which leap to mind are that the Dorsets are not seasonal breeders, so you can have a crop of lambs whenever it suits you. A lot of people lamb them late summer / autumn to get the best prices for 'spring lamb' after New Year

And people seem to get very
very attached to their Teeswaters. They had a couple at Woolfest last year, and they were the undisputed stars of the show. Take that, you alpacas
