BA do own an ambulance, but it is based at the engineering base, which is several miles from the terminals. It is required by UK Health and Safety rules, due to the large number of people working in the area, and the type of work that they do.
Although primarily for BA employees, they will respond to any emergency when requested to do so. Any incident requiring emergency assistance is co-ordinated by the BAA and the London Ambulance Service. They assess the availability and response times of the various facilities that are available and react accordingly. If they decide to utilise the BA ambulance, then that facility must be immediately replaced by other resources.
I gather that in this case it was decided that the fastest reponse, considering the location and the facilities available, was by the London Ambulance Service.
I have spoken to the BA paramedics. They are professional, caring people, and are deeply offended by any suggestion that they would refuse to respond to anyone in need.
Airclues