I am very biased, but would recommend Ryelands as they are not as large as most sheep, and due to short legs aren't keen on escaping! We have 12 and they are lovely, though some shearers aren't keen on them as they have copious amounts of wool! They lamb easily on their own and can have twins easily.
They have not long come off the rare breeds register, and so have made quite a come back.
However, there are other breeds to consider - Shetlands are very petite and sweet. Lots of members of this forum keep different varieties of sheep and will be able to give you more of an insight!