This voucher comes with its own set of terms and conditions and they are not displayed on avios.com when you are making a booking using the voucher. basically, once you use this voucher, it is gone for good. no 24 hour cooling off period, no nothing. if you cancel your reward booking, you will get back your Avios and taxes (with the £35 applicable fee) up to 24 hours before the flight but the voucher is gone. I believe this is in contrast to how BA Amex Premium Plus 2-for-1 voucher works. So you may want to be careful when using this voucher.