Whippersnapper says: "....why would the fuel freeze during the approach, 30 minutes or more after starting descent and the fuel beginning to warm up again? Surely fuel temperatures are at their lowest at the beginning of the descent, not the bottom of it?"
Pretty simple stuff.
Any mass of ice forming in the high-level cruise and adhering to a surface would, in a gradually warming environment of a descent, firstly become dislodged,move with the flow and then lodge somewhere downstream in the fuel-flow and cause an obstruction to that flow.
That simple deduction is a direct pointer to the true nature of BA038's problem.