I recently had cause to travel back to Canada from Heathrow on Air Canada as BA could not get me out during the Air Controllers' strike when BA also had a computer issue.
I now remember why I stopped flying Air Canada even though I have more Aeroplan miles that I know what to do with.
The shared lounge at Terminal 3, Heathrow is definitely not a great lounge. Shared with SAS and Turkish Airways, among others the food is buffet style only and there was nothing worth eating in the entire lounge. The staff are very nice but there is no service at all although the cleaners work non stop and do an excellent job.
The Air Canada business class (there is no first class at all!) is one of the least impressive that I have flown in for many years. As one of my friends commented to me (he travels to and in the U.S. with great frequency) -"Air Canada is a giant amongst midgets in the US". They are very short indeed when compared with world class airlines.
The flight staff, although friendly enough are not very good at all (the purser kept dropping bottles and anything else). They couldn't push the carts through business class without bumping into every seat. The washrooms were clean enough but the seats were 1/3 empty so a full flight would be a different issue.
The flight took longer than any flight I have taken from London to Toronto since the Icelandic volcano and for many years before that. My guess would be that Air Canada has told their pilots not to burn fuel and to fly with the throttle 1/2 open.
Not a great experience although we did get back, eventually.