I understand. I did try to call BA to ask if my homebound flight about a week in the future could be changed to one departing almost immediately, but after getting cut off twice and put on hold for 45+ minutes once, I gave up and cancelled via the app. I am sure the Avios and cash representing the surrendered return flights are gone for good. And, yes, the booking used a 2-for-1 voucher, otherwise I would have reserved the trips one leg at a time. Such is life.

UPDATE a week later: I've had a cash credit from BA post to my card account in the odd amount of US$335.60, about 60% of the surcharges-and-taxes portion of the cancelled booking, with no accompanying communication / explanation as yet. So the cancellation wasn't a total loss, just a significant one.