Thanks Betsy. I have checked with the travel agents and I can get an AC flight to YUL to connect with QR from some Canadian airports.
Or else as you say, I have to buy a separate document for my domestic sectors. Since the timings of QR is not bad for a YUL arrival, I do not mind paying a separate ticket for the domestic sector or else can fly the YQM LHR on AC and then QR from LHR.