A perhaps valid point has already been made if this B787 flight is the flight that you booked and paid for, ie regardless of the aircraft type is it still the same flight number and operating to the same times? ... Down my way there is a notoriously bad loco here where it is in their T&C's that should the flight become cancelled then one is entitled to a refund, a few times now I have booked way in advance to achieve silly fares for plans to then change, can't cancel but then the loco changes the schedule by perhaps as little as 10 minutes, "Aha, got you" and sure enough I have been allowed to cancel and received refunds.
But I note on BA's website:
"Apply for a voucher
If your flight has been cancelled you can claim a voucher to the value of your booking. Your voucher will be valid for travel until 30 April 2022 and can be used as payment, or part payment, for a future booking.
Your new trip booked with your voucher must be fully completed by 30 April 2022 (departure and return). You will receive your voucher by email within seven days of your application"
Now that isn't in the spirit of things, that one can spend their hard-earned money buying a product for the seller to withdraw that product from sale for the seller to only offer an alternative product that one doesn't want and with no refund of monies offered, I've had a couple of previous problems with the World's Favourite myself so I understand the brick wall mentality that you are dealing with, I'll never utilise their services again but everyone to their own, and I doubt you'll ever manage to change that brick wall of a supposed customer service.
This is all on the presumption that the flight no's and/or the scheduled timings may have changed, credit card has been mentioned but those are subject to credit agreement laws which is kind of a different subject, I pay by debit card, indeed with a foreign bank also, and it is possible to make a 'chargeback' claim thru a debit card payment, if you and your wife genuinely can't travel then your monies are presently lost anyway so you have nothing to lose by attempting a chargeback.
With my bank I need to complete a form, ticking boxes and all that, and one of the areas I need to select between is "goods not received" or "goods not as described", or similar, your purchase was for two flight bookings on those dates and at those times, should BA have changed those flights and times then your applicable selection would be "goods not as described" with perhaps a stumbling block being that you agreed to there being no refund whatsover available in the T&C's at the pre-COVID time that you booked.but it's worth a try.
Good luck
P.S. This link makes for some interesting reading:
British Airways: how to cancel, rebook or get a refund on your flight ...
"If your flight is cancelled, you are entitled to a full refund, as per existing British Airways policies."