I would call them again and tell them what you were originally booked for, what it was changed to, and what it is now. Tell them that you have important meetings in TO the day of the flight, and important meetings in the UK the day before, and that the flights you were changed to via FRA were perfect for you and your schedule after the previous flight was cancelled.

Sometimes it's just a matter of getting an agent that knows what they are doing. I have heard them say that it's on a manager's desk before, but I never heard back from this manager. So try calling in again.