If you know what you are doing you can use Charollais tups in outdoor lambing systems even in upland Cumbria.
I wouldn't go more than 50% Charollais (ie, I wouldn't use the Charollais tup on a part-Charollais ewe), and it's important to use a tup who isn't too bare and doesn't produce lambs which are too bare, but it can be done.
We used two Charollais tups, grandfather and grandson, over a period of five years, on our commercial sheep, lambing outdoors in upland Cumbria. We have kept on many of their daughters who do a grand job in our flock.
If the weather is cold and wet at lambing time, then the lambs will benefit from the clear plastic jackets, but although the lambs have thinner fleece than their Texel and Dutch Texel x peers, they are so active they are warmed by getting up and getting mum's milk quickly, which means less of this type get brought in for de-chilling than do the Texel types.
When we were first looking into Charollais tups we were told to buy one with a lot of wool on his head; this usually translates into having lambs who are woollier than some of this breed. Both of our boys had thick carpet on their noodles, and in general their lambs, though thin-skinned, were not dangerously so. We did get the odd pair that were really bare, generally from thinner-skinned mothers, so these families might get brought in if the weather was particularly evil. But generally the very active lambs, having had an easy lambing, were up and at the milk bar very quickly, and did well. If the weather was wet and cold we would jacket new-born lambs just to help them along.
I wouldn't have too many qualms about crossing Charollais onto Ryeland, in fact I think it would be a good mix. And the Charollais on the Dorset Mules would give good commercial lambs too.