I think it all depends on how much and what kind of land you have got. If only an acre or two, breeding sheep would be quite difficult, but you could rear a couple of lambs for the freezer quite easily, then rest the field until next summer. If you want longterm sheep (and are interested in the fleece/wool), then having a couple of adults (preferably wethers) would give you lawn mowing plus wool, but would need winter care (hay, feed etc etc).
Also before you decide on the breed think exactly what you want from your sheep - if you are only interested in meat than a sheep that doesn't need shearing is obviously a good idea.
Shetlands are great (mine have yet to try and jump the fence), and there are quite a few small flocks around the country, so sourcing good stock should not be a problem. Hebs and Borerays may be quite difficult to get hold off, unless you know of a local breeder. Shetlands are also a good breed for someone keeping them on their own, I can up-end my Shetland tup quite easily, haven't tried the Texel yet... (and he is only a tup lamb...)
I have a small flock of white pedigree shetland (I like to play around with natural dyes) for wool and meat for our own use, plus now a flock of commercial ewes plus texel tup to pay for my non-profit smallholding animals/hobbies.
We currently have an agreement with a friend/colleague and we are rearing two weaners and he will buy one of them at the point of slaughter. Means that I have an outlet for my goatsmilk and we only need one pig for our own needs. Not sure I would want a fully-fledged Co-op, as I like to keep my options open....