Don't waste your time arguing with BA. BA continues to assert that the Trent issue is an "extraordinary circumstance" and CEDR continues to determine that it is not. Simply send a clear note to BA asserting that it is wrong and asking for it to either relent and pay out the EUR 600 per passenger or to advise you of a standstill. Specifically reference your intent to proceed with a CEDR arbitration on some specific date, e.g. 10 business days. If BA pays up, all the better. If it does not, you will simply have to push papers, but unless CEDR changes its view, you will have your decision and thus funds soon enough.
Why does BA continue to litigate this issue? Likely because most claimants do not pursue their claim when denied. It costs BA very little to go through the make work of defending the Trent claim. If most claims drop off when declined, that is a net win for BA.