You are right that D should be semi-flexible and Y fully flexible. Hence D would be cheaper than Y.

However, fare rules quoted for LGW-TRN seem to be same: change at anytime subject to any difference in fare and cancel at any time. Both are without fee or penalty if done on line.

I don't get this either. I thought D permitted changes at a cost and no refunds, and only J allowed refunds without charge. Seems odd to me. It would mean all the benefit of J potentially at a D price except if you wanted to change where on J fares were left. How odd.