Well, I can't speak for AIR CANADA or any operational concerns that the company may or may not have. I can only speak for myself as an A.C. B-767 captain who has been a frequent user of LHR and other U.K. ATCO services since the '70s. This sounds like a non-issue to me. The system works very well.
We call for start-up, we get our clearance, the SID altitude is set and the pre-loaded SID route confirmed by both or all 3 pilots in such cases. We change frequencies, request push and start.....and away we go.
It works the same way all the time, except of course if there's a slot delay, which occasionally happens and which we usually have prior knowledge of.
It's an excellent system that is predictable and works very well. Don't see any 'safety' related issue there. IMHO, of course.