I was recently upgraded at the gate (thanks BA!) on an LHR-GLA flight. I had no checked bags so had reached the gate without the need to show ID. Gate agent asked me for my passport for the new boarding pass - cue a discussion along the lines of "why would I have my passport for a domestic flight", "it's BA rules", "no it isn't", "yes it is" etc etc. Couldn't be bothered arguing further and potentially delaying others, so dug my driving licence out.