An airline employee holding a passenger's phone and/or passport for leverage or any reason unrelated to critical official duties such as scanning a mobile boarding pass or checking a visa is completely unacceptable. Personal property can now be held hostage in a very minor baggage dispute? Unbelievable.
I don't fly BA that often, but this stuck out to me. If a GA would not give me my phone or *passport* back over a bag, I would politely ask for a supervisor to solve the issue.