On the point about the carrier surcharge, clearly it does differ depending on your class of travel - after all, they paid you £28 as downgrade reimbursement (on the purported basis that they just have to pay you the difference in YQ - which is obviously wrong).

Having evidence that the carrier surcharge varies by class of travel - or rather, BA's inability to prove otherwise - was the point that won my relatives' CEDR case which ended up just being about the YQ.