Yes,YYZ has been operated before by AI,and it was fairly successful.
The reason the flight got axed was because AI were given an ultimatum by the powers that be to use LHR slots or lose them.If you remember the flight was effectively transferred from BHX to LHR,although if memory serves it didn't actually last very long.
Whether the slots were retained by AI after that for another service at LHR,I can't remember,but as we all know AI returned to BHX and the rest is history.
What most people would like to know is whether the YYZ service is extra to what we have now,so 2 flights a day on the days of operation,or whether it is a tag on to the current service.DEL-BHX-YYZ-BHX-DEL on a daily basis.I am guessing a lot will depend on aircraft availabilty,but we do know some more 788s are due in the coming months.