It was probably me that had a similar question in the summer. There were 3 of us booked on a BA booking London - Doha - Sydney on Qatar returning Brisbane - Singapore- London with Qantas and BA. I couldn't board at Heathrow due to a visa issue so the other 2 went on without me. BA customer services told me several times that my return would be cancelled automatically. I flew down on a new one way ticket with Thai and caught up with my family. I kept checking the booking on the BA website and it showed as live for all 3 of us. We checked in at Brisbane and everything was fine.