And I will add my welcome too,
deckem. Montréal is one of my favourite cities and I hope you will have a good flight to London from there. Welcome to the BA forum and I hope you will have a good time exploring this resource. Bienvenue à bord.
One thread in particular is useful:
http://www.flyertalk.com/forum/briti...ller-plus.html
and in addition to the tools mentioned by
evanderm, you can check the seat diagram on MMB 2 days before departure and that gives you more clues.