MarkD - Don't worry. EI and BA have a very strong alliance and EI will honour BA's higher allowance for the ORK-LHR sector and vv. Just ensure the EI check in agent sees your onward flight coupon at check-in. If you're fortunate ('cos not all EI computer systems are compatible with ours yet) he/she may even be able to check you in for our LHR-YYZ flight and tag your bags through to save you lugging them between the Terminals at that funny airport around the M25 from Gatwick (hub of the universe!)
Now, for passengers travelling with airlines outside alliances things become a little more complicated!
If your travel is all contained in the same booking and on the same ticket or conjunction tickets and your ticket coupons are all being used in sequence and your connecting flight is the first available service (ie you're not stopping over en-route), the the heavier allowance applies throughout your journey!
There is. currently, an IATA agreement that scheduled carriers will honour the 2 X32kg allowance to/from Transatlantic destinations on connecting services. (How long for is a mystery, given the huge volume of Delta, NorthWest, US Airways and Continental connectors we seem to be getting lately who have only paid about $20 for the onward sector!)
Be very careful though - it has to be genuine connecting services contained in the same booking! The "no-frills" carriers fall outside of this agreement and would probably rub their hands with glee if you appeared with 64kg of baggage at the thought of how much excess they could charge!