T5 is squeezed between the two runways to the north and south. The m25 to the west, and the central area to the east. Heathrow is by modern standards a very small airport in area. That small space meant the architects had to go "vertical" with the design of T5. Hence the lifts and elevators that everyone hates.
T5 was never going to be big enough for BA. It cannot be any bigger because of the space constraints.
If the UK was China we could bulldoze most of Staines away...
So the quote below seems a little over the top.
What kind of moron (& I use the only because I can't think of a stronger one) has a billion pound terminal built that isn't big enough & has no room for future expansion? The BA board members who agreed to this deal must have the collective brain power of a six week old boiled cabbage.