If AC doesn't want to rebook you (without a fee) until the flight is actually cancelled, I suppose the best you can do is keep an eye on the flight and have them re-book you as soon as it is actually cancelled, then you will probably still be ahead of most other pax.

Otherwise, would it be possible to make a new booking (if the same price is still available) through LHR and then get a refund on the original one when AC cancels your original flight?