I don't know about prices - or availability - in the south of England, but in southern Scotland, Soay and Castlemilk Moorit wether lambs fetched less than £20 at Lanark sales. You'll pay a bit more going to a breeder, I would think. I once paid £50/head for Castlemilk wether hoggs (eighteen months old); that was pricey, and they'd be cheaper as lambs of course.
I don't know if there are likely to be any coming through the ring at sales in the SW. Sedgmoor do rare breed sales, but wethers might be sold in the regular store sales.
I would suggest you think of getting more than two, of any breed. Sheep are flock animals, and really do need company of their own kind. It is said that less than four sheep isn't really a flock, but in any case we always say get at least three, then if something happens to one, the other still has a pal.
You might find you need a bit of hay for them after Christmas when there's no goodness in the grass, but three or four Soay would need very little, easy enough to buy by the small bale locally from another sheep keeper or a horse owner. A small bale would last four Soay ten days or more, I would think. (Or you could get yourself a scythe and a rake and make a bit of loose hay in the summer when you've got too much!)
You would still need to be prepared to check them every day, protect them against fly strike during the warmer months, or be certain you would spot it and react quickly should they get it, keep an eye on condition and worm or fluke as needed (and or get faeces tested to check for worms or fluke, and treat accordingly.). If you know other sheep keepers locally you could probably ask one of them to give yours a once-over every month or so, in addition to your daily checks, just so that someone experienced would notice any issues.
Other than that, they'll need good fencing and clean fresh water.