From BA website: "You can easily use your eVouchers on ba.com to pay or part-pay for flights that depart and return by 30 September 2023, so there's no need to call us. Up to four eVouchers can be used per booking. If your total booking value is less than your eVoucher, we'll automatically send you another eVoucher for the remaining amount.
To redeem them, simply select your flights on ba.com and continue to the 'Review your flights' page. Click the link at the bottom of the page and apply your eVoucher details. You'll need your registered surname and email address associated with the eVoucher."