Air Canada are part of Star Alliance so you should be checked straight through to your destination airport. You'll get both boarding cards when you check-in so no need to collect your bags at LHR, just proceed to your connecting flight.
Dazbo, that would have been correct a few years ago.....I remember the furore in the bmi forum at Flyertalk when they changed their policy. bmi will only now interline if all flights are on the one PNR. If separate tix are involved, bmi will not interline. http://www.flybmi.com/bmi/en-gb/planningandinformation/check-in/throughcheck-in.aspx
Buster - you might have better luck with AC checking you and your luggage all the way on your return journey. Please ensure that you leave enough time at LHR to collect your luggage and check in for your onward flight.