I'd say that TravelBuzz is as good a place for this as any - while the OP may intend to fly on BA this is really a more general question and the answers would be the same regardless of airline flown.

Once your baggage is in the system you won't be able to retrieve it. It's a question of logistics - there are thousands of bags in the system at a large airport at any one time, there is no way they could go and look for one specific bag.

You could try asking staff at YYZ to tag your bag to LHR instead of CDG, but what if they say "no"?

If you check your bag to CDG, and to don't get on the last flight, it will eventually get offloaded (after they've put out several calls for you and held everybody up). What they would then do with it I don't know, but you might lose it altogether.

I'd either travel with hand baggage, or think of something else - like booking CDG-LHR on a separate itinerary (you could always buy a cheap return and throw away the return coupon).