秋海棠,
My girl has experienced UM service from Qantas (free) and BA (charged) ex HKG and ex LHR this summer. Both of them offered the similar service. The ground staff will bring UM to pass through immigration...on board of aircraft. Then flight attendant will take over and pay attention to UM in-flight. Passport and boarding pass will be kept by them until arrival. Again ground staff will take over... and bring to arrival hall and hand over to the registered person for picking up.
Don't worry as it is quite safe.