Welcome to FlyerTalk!
The best place for your query is the FlyerTalk BA forum. I'll alert a Moderator and request that this thread be moved there.
How long ago did you purchase the one-way ticket? Was it a Basic Economy fare (hand baggage only), or a "regular" Economy fare? Did you ask the BA agent whether the ticket would have any residual value if you cancelled and rebooked?
Good luck!