score:3
The UK does not have exit border checks, so you do not have to show your passport to anybody.
You do have to show a passport to the airline operating the plane you are boarding (or train or boat operator), but that is solely to prove you have the right to enter the destination country. They do not care about the country you are leaving, and certainly not about how you entered it. Any passport that will get you into the destination is acceptable.
As a UK national I have entered and exited the UK on a Canadian passport many times.
Upvote:2
My kid has a dual citizenship, Austrian and Russian. When we leave Austria for Russia, we show the Russian passport at the norder. When we travel back to Austria, we show the Austrian passport. So in your case you'll probably have no issue at all with leaving the UK with your Canadian passport (depending on where you're going, though), but how are you going to come back later?