Hello, we're traveling SEA to LHR in October, and LHR to SEA in November 2023. I've just realized that one member of our party has a US passport that expires in March 2024. Will this be a problem for him boarding our British Airways flights? Does he need to renew his passport prior to our departure? I don't know what the rules are as to how long his passport needs to be valid beyond our departure/return dates. Does anyone know the rules for certain?