I have to agree that the queues at Gatwick are really bad. I was flying to Ireland one day, and there were at least four other flights using the "Ireland and UK flights" channel. It took about 45 minutes to get through, and people were sailing through the international departures, which really annoyed everybody who was waiting to go to Ireland.
I've also seen Stansted with very long queues. It seems as though the number of security channels that are open is in inverse proportion to the number of passengers, which can lead to problems, especially in the summer.