It is called married segment. You may not be able to drop the segment at all (when you drop the segment the LHR-JFK availability disappears). However if you can, there is no fee based on the new rule in effect from Feb this year - when cancelling the last segment does not affect the nature of the award - in this case it doesn't because LHR-JFK is still Europe to North America.

Or, you could fly the itinerary and skip the last segment provided your bags not checked thru or not checking bag at all.

AA does not fly LHR-YYZ direct, right? So you should not have the re-route issue in case your LHR-JFK got irrop because I think AA would be reluctant to accommodate you on BA to fly LHR-YYZ. So you would always route through the US East Coast anyway.