Follow-up: US BAEC agents said it couldn't be done (2 of them). I called the UK BAEC and the agent said "sure, why not?" Except the US BAEC has to do it for me.
Called the US BAEC the next day and a 3rd agent said can't be done. I pointed her to the notes in the PNR from the UK agent who said it can. She said one second and called to UK BAEC to get it sorted out... and by sorted out I mean transferred me when she had someone on the line who then proceeded to have no idea why she transferred me to the UK when they can't do anything for me but that, yes, it's completely valid to cancel just the outbound. He transferred me back to the US BAEC and a *fourth* agent told me it can't be done but she was willing to go look into it and sure enough... done.
I didn't consider the APD was so high even for coach fares (I guess that's what it is?) because I had a substantial refund of taxes and fees despite the $40 change and $25 phone booking fee per passenger ($260 total refund *after* change fees!, plus 25,000 avios back).