I have no idea why Lovablemrkno regards this as a trick question, but then I never know why he posts anything anyway
You are not required to present a passport when travelling within the UK (or between the UK and the Republic of Ireland). However, all airlines require that you present some form of identification. The requirements vary between airlines. All airlines accept a valid passport or an EU photocard driving licence. The stricter airlines (e.g. Ryanair) won't accept anything else from UK citizens but easyJet will accept almost any form of photo ID (e.g. a bus pass).
If you've got a passport, you might as well take it,because you'll then definitely have valid ID.
Chris