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Just wrote an epic post on overwintering sheep, and Cloudflare then lost the lot.
I'll try again (this time! I will put a copy in paste buffer before pressing "Post"!)
Check your abattoir can take home kills in December / January. Ours won't, and any for late Oct, Nov, or early Jan need to be booked in months in advance. (Like, in July.)
If your priority by December is the meat, and you can get a slot at the abattoir, I'd go for something that'll be ready in December and send them off then. Any sheep will stagnate if not lose condition once the grass loses its nutritional value.
As a generalisation, commercial types will lose condition once the colder and/or wetter weather comes, unless supplemented and given good dry shelter.
If you'd rather have them grazing for longer, then you need something won't go backwards and ideally can grow through winter on just forage.
The traditional choice for overwintering is Cheviot. Lambed in late April / May on the hills, sold as weanlings late August/Sept, at that point looking like bunny rabbits and fetching more than fat lambs three times their size in the primestock ring next door. They make good use of anything and by New Year are impressive "butchers lambs" fetching top dollar as "spring lamb" until the first of that year's crop hit the marts. Dunno if you'd get any around here though, and they usually don't get sold in small numbers. If they do go through the marts here, you might be able to get one of the buyers to sell a small handful of his/her purchase on to you at the mart.
Primitives like Hebs and Shetlands will keep condition in the circs you describe, but won't grow much once winter hits and would be a lot bigger - and tastier - after a second summer.
Over the border in Cornwall, around Bodmin, there are lots of Herdwick crosses and Swaledale crosses, which would cope with the circs you describe. Pure Herdies traditionally get three or even four summers, and Swales two summers, to develop the trademark flavour, but crosses might work for you. (But avoid ones that look like Mules, they'd likely be too soft to be putting anything on over winter.)
I'm not as familiar with the sheep on Dartmoor, but I'm guessing there will be Blackfaces and crosses thereof, which would be similar to Swales and crosses, and should be a bit meatier.
I don't have personal experience of Devon & Cornwall Longwools, but there are plenty of them, and crosses of them, around Bodmin, and they should do what you want I would think.
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. She even laid a few eggs in her final year (distinctive from all the other hens so knew they were her eggs). It was truly a sad day when she went. 
