Will the fare rules allow a stopover (more than 24 hours) in London on the return journey? Or will they allow you to fly YYZ-LHR then "connect" to an LGW-CDG? Either way, it allows you legitimately to claim your bag at LHR, because that's as far as it will travel without you claiming it.

If you want to try your luck at this, the best thing is to not only fail to show up for the LON-CDG flight, but to fail to check-in for it as well. This is not only for your own benefit. Even if you're going to play the system, please don't check-in and then fail to show up without warning - spare a thought for all the other people on that LON-CDG flight who will be sitting there getting more and more delayed while the airline frantically pages all round the terminal looking for you.