A lot of UK airports, especially BAA seem to have a policy of photography whilst airside. I dont know why that is. I once produced a camera whilst rampside at Stansted and the handling agent went berserk and said "dont do that" as BAA were likely to come racing out and have words.
Other airports seem generally OK, however I feel it is best to seek permission first for formalised shoots. You are right, nobody is EVER going to stop snapshots - specially in these days of cameraphones etc.
The Friday T5 tours for the public are the best, NO CAMERAS, they are NOT allowed on the bus.