If there was duty-free that you wanted to bring as gifts for those in NS, then going through LHR makes more sense since you'd have to pack the duty-free in your checked bags once you get to YYZ or YUL.

(And I'll second the comment that YHZ is easy from a Customs point of view.)

(LHR-YHZ is our common routing home to YOW because we stop to visit family after business trips overseas. It is cheap - last time was only $8 more than flying straight home to YOW - and the flight is a short enough that you get a several hours of visiting in on arrival day. Plus, it isn't a busy route so upgrades are easy to come by.)