You cannot use the 24-hour cancellation guarantee to cancel a flight booked within 24 hours of departure and receive a full refund.

You can cancel a redemption booking at any time, but if it is cancelled less than 24 hours before departure and the above does not apply, you will only receive back the external taxes and charges (and not the Avios or YQ).

I strongly recommend not waiting until the airport as (a) reward availability for your preferred date may have gone and (b) the airport ticketing staff may well have no ability to do anything with reward bookings.

For future reference please be aware of the B in BA: it uses British date formats This is a feature, not a glitch.