Can you just clarify what you are trying to do? I am reading it as flights from YVR-LHR-BCN with a hotel in BCN, then you make your own arrangements and a while later you fly Milan-LHR-YVR? Assuming this is the case, you'll be on a single ticket and the open jaw won't matter too much.

If that's the case, then I believe you'll be protected on the outbound as long as you arrive in good time at YVR, so any delays YVR-LHR become BA's problem but your inconvenience. For the return as long as you're at Milan in good time for that flight, they will be obliged to get you to YVR even if the Milan-LHR is delayed. Conversely, should you not present yourself for the Milan flight, your return would become your problem, not BA's.

Key thing is to be there at check-in at YVR on outbound and Milan on return at designated times.