Don't know how much you want to avoid LHR, and I guess you want to use cheap staff travel, but Zoom fly direct from Canada to Glasgow.
Also you could connect in the US instead (e.g. via Newark (Continental), Philadelphia (US Airways) - shorter travel time than going via LHR.
Otherwise I'd go with the suggestion of BMI from Heathrow. BA have a tendency to put their domestic shuttles at the bottom of the priority list when anything goes wrong at LHR.
And road and rail traffic around London is such that I wouldn't recommend going to a different airport.
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