I am inclined to go with the description of what they were sold as.
I really do not think there is much if any Black Welsh Mountain blood there. The shape of the head is wrong, the appearance of the fleece is wrong, and one of them isn't black which is fairly unlikely if the other half is Southdown.
The heads do look a little like the Dorset Down heads of my youth. The variations in colour may well indicate Shetland.
Whatever they are they are nice and obviously very happy with you.