When deciding where to operate the A321-XLR, the first question that AC will ask will be "Can we consistently sell 14 J lie-flat seats at a premium on this route?" If the answer is no, I would expect them to use the 737 MAX instead or not fly the route at all. Those J seats take up a lot of space, and someone needs to pay for that space.

YOW-LHR and YHZ-LHR are likely routes. Maybe YOW-FRA. They will probably use them for some secondary European markets from YYZ and YUL that have a strong demand for J but where they struggle to fill up a widebody year round. I could also see them on transcon routes (YYZ/YUL - YVR, SFO, LAX, maybe YYC).

It is unlikely that they will operate them on primarily leisure routes. And unfortunately for YWG, I can't see them selling enough J seats at $3000+ each to justify flying from there to Europe, or to make it worthwhile to dedicate an LHR slot to the route.