Someone will pull up the T&C's but you really think they will pay you new flights or extend validity of your voucher because of risk of bad publicity? They have a business to run and I don't think the public will be so symphatetic towards "we planned to travel both in FIRST on holiday for 350 pounds pp using our airmiles and BA Amex voucher, now we cant make it and BA does not help us". You'll get your money and miles back and I think that's it.