I think you need to try to find someone doing the return journey, or part of it, so glad of a load in the other direction.
What livestock gets exported from Yorkshire to Wales? Actually, Welsh Country Foods (now part of Vion) and Dunbia both have abattoirs in Wales, and are bound to pick up near you. But not sure whether they'd already have return loads they routinely do and if not, whether they'd countenance picking up just a few sheep for you. Can't hurt to ask, though?
Somewhen I am hoping to get some Exmoor Horn ewes, probably from Exmoor. Luckily I do know of a Devon family who do occasionally buy Galloway cattle and Beltex sheep up here and one of my neighbours arranges the transport for them, so I may be able to hitch-hike some sheep on the otherwise empty journey.