Very easy! Once it gets added to your BAEC account, it's a radio button to select to use it on a reward booking. You still need 2 x seats available for redemption, but the voucher is considered when pricing up the Avios required. We've talked about cancelling cards before, and on my last cycle Amex were very persistent that I keep the card (downgraded to free card OK), otherwise the voucher is cancelled. Part of me thinks that still isn't true, but I wasn't prepared to risk it. Be interesting to hear from anyone who did cancel their BA Amex and keep the companion voucher.