I can't see the point of these if they don't actually properly search. Was at the Natural History Museum today, and the bored looking man asked me if I had a pen knife or scissors. I said no, he barely looked in the bag and didn't look in the tied up carrier bag (happened to be of dirty clothes!) at the bottom of my backpack. I could have carried anything in. Also teenage foreign students didn't have their bags searched.
It meant the queue to get in was about an hour long as they only had 3 people checking bags.
DH thought the search was because royalty were on site, I reckon they do them all the time though!