We've tried Lleyns but too many triplets and worse, and it's tough for any sheep to rear three on our ground. Most years we can keep the mules' output reasonable by not flushing (the opposite of flushing, in fact) but this year everything's having nothing but triplets due to the extraordinarily good long summer.
The Swaley Mule can take any tup, lovely roomy ewe, mostly fairly placid to handle, fantastic mothers, usually plenty milk. I can't imagine how they were found to be not hardy enough for South Wales - what mothers were they from? For very hardy mules, you have to use a BFL with not the very finest of fleece and a mother from a tough location and a fleece to go with that
They will need feeding through pregnancy, mind, which some farms don't want to be doing.