I would call BAH as soon as you can (the hard bit will be getting through to them on the phone!) I found them pretty flexible a couple of times last year when I needed to make changes which weren't necessarily strictly within their T&Cs. Appreciate you'd rather change the destination than get a refund, but my reading of the following link is that you could get a refund if the foreign country are denying entry:

https://www.britishairways.com/en-gb...oucher-request

"I’m travelling within the next 28 days and have been impacted by one of the following that cannot be avoided through testing or vaccination:
Restrictions that prevent entry to the country or mandatory quarantine have been applied at my destination"