I don't think it quite works that way. You're going to eat extra the YQ anyway since the origin is YVR.
A open jaw YVR-LHR/MXP-LHR-YVR in F/CE is 726 CAD YQ.
Conclusion? BA does what they want. They could make the F fare $1 and $15,000 YQ in one direction and the fare $15,000 and YQ $1 and you could not absolutely nothing about it.