My sister didn't forget her passport when she checked in at Heathrow to visit my brother in Annecy a decade or two back. She explained to the BA desk staff that she didn't need it as she was flying to France which is in the EU which has abolished all internal border checks. "You still need your passport when in France" said the agent "and in any case you are flying to Geneva which is in Switzerland which isn't in the EU and hasn't abandoned any border checks". She was so woebegone they transferred her to the next day's flight with no charge...