I was wondering whether in the event of BA wasting everyone's time by quibbling about voucher value, there might be some recourse against Amex, since of course the extra GBP250 for the Premium Plus card is largely justified by the Business vs Economy entitlement. The Amex wording is clear on what is being sold with this card subscription:

"Get a Companion Voucher when you spend £10,000 each membership year on the Card, which you can use to take a companion with you in the same flight and cabin.Vouchers are redeemable when you book a British Airways Reward flight in any cabin."

In these circumstances, I would certainly ask the card seller for either a replacement voucher or a full card fee refund. Hopefully Amex would not take kindly to BA trashing the former's product if there is any pattern or policy of targeted downgrades and/or rejection of due compensation.