From my personal experience having entered the UK on an expired passport for several years, you still need a valid passport of some description to travel with. Whilst I've never tried, I would hazard a guess that the airline would not allow you to board with an expired passport whether you'd be eligible to enter the destination country or not.

Whenever I did enter the UK on an expired UK passport, the officer would still ask to see my Australian passport as the airline could be fined if they let me travel purely on the UK passport.