As your voucher's expiry date is fast approaching and your previous attempts to use it have been thwarted, if I was in your shoes, I'd call the Customer Relations team tomorrow to see if they can either unlock the voucher code or provide you with a replacement together with an assurance it is validated by BA's online systems. Also, under the circumstances I don't think it is unreasonable to request an extension. I believe your best bet would be to call in the afternoon (UK time) when the CR agents are manning the phones.