If LHR-YYZ-ROC is cheaper than LHR-YYZ, then some people might tempted to purchase the former and abandon the second leg to save money and in theory, AC can recoup the fare difference. Of course your situation is different, AC failed to provide you the second leg and they should pay for your car rental charges + reasonable incidentals + whatever the value of YYZ-ROC. Although the value of the second leg can be tricky to calculate, especially if LHR-YYZ-ROC is cheaper than LHR-YYZ.