YYZ - LHR - BAH - PEW on the outbound? Looks like even if MSC rules applied, YYZ - LHR is the longest segment so AC is the most significant carrier... you should be entitled to AC baggage rules, regardless of which airline actually issues the ticket. Since you're E35K, you should be entitled to your E35K baggage allowance anyway as you'll be checking in with AC for the first segment.