I think prob is its not a YYZ or YVR - LHR return. Trying to finish in YVR is the challenge.
I just combined PY from YYZ to ATH with a stopover in LHR and P on the return to take advantage of the discount P fares ex-Canada but which (now) dont allow a stopover in LHR.
Starting in YYZ and ending in YVR will be tricky.