Originally Posted by
LondonElite
Only obliquely relevant to the OP. In your case, you had to show your Canadian passport because, as a Canadian citizen, you must travel to Canada on your Canadian passport. (We will set aside the fact that Canadian immigration would have to let you into the country regardless of your passport issue if they establish that you are a citizen, as you would never have had the opportunity to present yourself to them since you'd not have been allowed on the plane.) The fact that you were on a return ticket was not relevant to the requirement to show your Canadian passport.
But travelling TO London, England, surely the airline only cares that I am admissible to the UK (not that I can return to Canada on a different flight a few weeks later)? It's the only time I have been asked to show proof that I can take the return leg of a flight at a later date.