Years ago I was able to book a BA-Westjet combination on one ticket and BA could not issue a WS boarding pass, I needed to see a WS agent on the ground at my transfer point. I think WS uses T3 at LHR, it used to be T4.

BA and AC do have interline agreements, it's actually easier to do that than a WS connection. These days there is absolutely no way I'd consider a US connection, and it adds a lot of flying time. DFW-YYC is 4 hours, and who wants to fly 4 hours south to connect and go 4 hours back north? When LHR-YYC is only 8 hours?

However, in this case I think it's no brainer - KLM all the way with a connection in AMS. WS isn't what it used to be either, after flying both WS and AC recently, I would choose AC every time now. It used to be the reverse but WS also has a costcutter as CEO. The WS forum is full of unhappy people now.