Originally Posted by AJLondon
T3 is used by BA so as to ensure they can have a say in the running of all the terminals at LHR. Thus the MIA flight allows them to attend and contribute to the T3 meetings etc as one of the operating airlines and hence decision makers out of T3.
That's not quite the whole story. Have a look at a bird's eye view of the LHR site and notice the proximity of T3 to the new T5. Then you'll realise why BA keeps a presence in T3 !!! ... no runways to cross !!