Remember people I wrote,
"First, let me say this post is for information and amusement only, it is not intended to suggest anyone do anything."
Take it that way. It's not intended to get anyone bent out of shape about it.
Nrr, I would produce a valid passport that allowed me to enter France. If both (not likely) my Canadian and UK passports were expired, I would produce my Swiss passport as my Cayman Island one would require me to have a Schengen Visa.
TemboOne and encampbe, I mentioned the airline having a different requirement might be a hassle. I have no doubt you might have to quote them the Immigration law of the country you were flying to. An expired passport does indeed meet Immigration entry requirements for Canada and the UK. Therefore the airline has no fear of liability but yes, you could spend time getting the airline to accept that fact.
As I wrote, I have ENTERED on an expired passport. I have never tried to LEAVE on one. But THEORETICALLY you should be able to do so.
TemboOne, thanks for the words on UK passport renewal. I don't live in any one country and renew them in the country they are issued from.
Often1, try entering Canada without proof of your right to do so. You have to prove it somehow. A birth certificate, a citizenship card, a passport. But having done so several times with an expired passport I can tell you that not once did I get taken for secondary inspection. What happened each time was the Agent simply asked the usual questions and then commented, 'you know your passport is expired' and I replied, 'yes, I know, I plan to renew it soon.' That might surprise you a bit.
Theassassin, given your name I guess it comes naturally to you to suspect that kind of thing. As others have indicated to you however, it is not unusual for ordinary people to also have several passports. I have four. I could tell you how I acquired them but then I'd have to kill you of course. ;-)
Mandolino, you are correct that a country cannot usually deny entry to one of their own nationals. They can and do however require you to prove you are a national. That is what an expired passport can and does do. Otherwise it could become as nrr aludes to in the movie 'The Terminal' starring Tom Hanks. A stateless person stuck in limbo.