... I'm undecided about where I think CUK should stand on the matter, ....
Cycling UK is a charity and so is required by law to follow its charitable purposes.
Off the top of my head - I can't find anything on the www and I'm not sufficently bothered to search - these are "the promotion of cycling," or words to that effect. So, with regard to helmets, and compulsion in particular, they should be guided only by their effects on the promotion of cycling.
Is there any evidence that the availability of helmets promotes cycling? I don't know, but I doubt it.
Is there any evidence that compulsory helmet wearing promotes cycling? Here is where the evidence seems strongest and it's to the contrary, so a policy of "no compulsion" seems to me to be evidence-based, (edit to add) even though it was originally reached by the CTC for reasons of expediency, as I tried to explain earlier.
There is the obvious point that "common sense" based virtual compulsion is not much different from a statutory requirement, but how can that realistically be opposed? Would a decision by Cycling UK to oppose everything to do with helmets make a significant difference to that social pressure?