we have to remember it's BAA who called the shots.. EDI flights impact on GLA who were no doubt delighted without the competition.
Not to the extent you imply, EDI could happily take 2-3 more A310s to YYZ if the market was there to support it without harming GLA. Look at Continental, it used to be a summer daily DC10 at GLA and that now sits at one daily B75W after a peak of 11 weekly B75W. The market wanted EDI and now EDI has twice as many Continental flights as GLA. Delta didn't even try GLA, it was EDI that got two bites at the cherry with ATL and then again with JFK.
There have been no contstraints on EDI having long haul since 1989, some twenty two years ago. I recall Worldways used to operate EDI-PIK-YYZ on the Tristar and the DC8 when I was a kid but since the end of that policy under Cecil Parkinson there has been nothing to stop them if the market's there, frankly they've done very well ! Incidentally the EDI-LHR feeder operation is a fraction of it's previous prestige, a lot of the market uses EDI-LCY on point to point leaving LHR slots for long haul. I don't imagine you're annoyed at those pesky Air France, KLM and Lufthansa services feeding CDG, AMS and FRA?
Some perspective please!