witchdctor
There is no requirement to have a valid passport on a BA Domestic sector. Photo ID is only required when collecting a paper ticket and does not need to be a passport. Many people travel on domestic flights who do not even possess a passport let alone an invalid one. Therefore there was no requirement for the Newcastle staff or security to check the validity of the passport.
It would be different if she had been through checked, but as there is no through checked baggage transfer between Heathrow and Gatwick, the Newcastle to Heathrow sector would have been treated as purely domestic.
I'm pleased to hear that the BA staff at Gatwick picked up the error, and that you were subsequently looked after so well.
The regulations can be found in the 'questions and answers' section of
www.ba.com
Airclues