Strictly speaking, I think that you are solely responsible.

If you miss the BA flight, it'll be because you've left insufficient time to get to YUL in the same way as if you'd left insufficient margin for the possible accident/jam on the M25 when taking a coach to LHR. BA would be entitled to take the view that you simply no-showed for the BA flight. Which bit of your public transport to the airport let you down isn't really relevant. And the consequences of the no-show could be that your ticket has no residual value save for a refund of unused third party taxes and fees. If it goes wrong, you may find that BA is prepared to be more lenient/generous in one or more respects. But that's something that you'd probably only find out on the day.

If you still think that 2:35 is plenty of time, then that's your decision. As I said, you might want to think about your options.