Update 2, in case this may be useful to anyone in the future: I called the BAEC line (UK line answered in less than 5 min around 10:50 today), and simply had the outbound canceled while the inbound was retained. Roughly 3500 Avios from the outbound credited back immediately and also due to receive a £32.50 refund in a few days. I'll now buy the outbound separately.