Capacity?
"Heathrow is groaning under the strain of 68 million passengers a year, about 50 per cent more than its capacity of 45 million"
How does an airport operate at 50% more than it's capacity? It obviously handling it (although not very well). It would seem Heathrow's capacity is in fact 68million a year. Even if they are all uncomfortable and checking-in in portakabins.