There was a guy from Easyjet on Radio 4 saying they'd cancelled flights "because the airports had asked them to", to prevent overcrowding at security checks. Doesn't sound like forces beyond their control to me, unless they're corporately incapable of refusing a polite request. Plus, it's been five years since society decided that terrorist attacks would be over-reacted to. Surely that's enough time to ensure that your airport has the facilities to cope with a major terror alert?