Really, it doesn't take a genius to figure out that the pax will end up at immigration and be held up there for some considerable time at best, or be deported (at BA's expense?) at worst. Supervisors will have been involved, and it would have been no trouble at all identifying the pax at the checkpoint and reuniting her with her ID card. The crew knew she must have left in the first bus, and there was another bus leaving. How hard could it be to give it to the driver and have a BA rep meet the bus? Or even for a crew member to join the bus and take it to passport control? Not because the pax shouldn't be more careful with her travel documents, but because it's the sort of kind and thoughtful thing that an airline with a human face will do. Honestly, looking at the replies the OP got there must be a lot of people spitting blood when watching the BA safety video when the FA picks up the cuddly toy dropped by that reckless irresponsible child instead of teaching her a lesson about personal responsibility!