A 747 landing at the Island or Buttonville would be an interesting site to see, I must admit.
Though the primary utility is for fares/schedules NYC - LON will give you more options than say JFK - LHR.
As for the Island Airport
You can book a YTZ - YWG schedule like this
YTZ - YUL
YUL - YYZ
YYZ - YWG
The flight time is 6h vs 2h for a direct YYZ-YWG flight insane perhaps but it can be done. This avoid taking the long ferry ride to the Island(NB the ferry ride to the airport is touted as the shortest in the world c.2 min)