Are you are a dual Canadian and British citizen? If so, you don't need anything other than your passport.

But if you are a British citizen, but a permanent resident of Canada, then you do indeed need a PR card to re-enter Canada. The airline is supposed to ensure that you have it at check-in for your return trip, and they are liable if they allow you on the flight without one. I'm always asked for mine, and the CBSA agent also wants to see it at the border.