All BA flights have 4 economy + 2 business seats released 355 days in advance (or when a route is loaded into the booking system if later). If not available, that means the LHR-YYZ seats for next May have already been snapped up.
BA may release extra seats if they feel like it (tends to be closer to flight date if it happens at all, but can be any time or never). However this is deliberately unpredictable in order to encourage people to book cash tickets rather than relying on reward seats becoming available.
Good luck!