THE BRITISH AIRWAYS WORLD CARGO CENTRE, HEATHROW     The distinctive metallic building on the south of Heathrow's runways, west of Terminal 4, is vast. Opened in 1999, it cost 250 million pounds sterling to build and provides 83,000 square metres, doubling British Airways' capacity at its main hub to 800,000 tonnes a year. The facility employs 1,550 people and handles an estimated £20 billion worth of freight, courier and mail each year. The engineers prevented cracking by designing a large number of SK bearings into the structure.