There is secondary RANDOM searches at the second check. Why don't you just ask for a water bottle on the plane? Why bring your own plastic container?
FYI get to Gatwick early, the security lines lately have been LONG. I got there yesterday a very unusual two hours early (normally, I enjoy cutting every flight too close for comfort) and I had no extra time. The security line was insane. I hear it has been backing up all the way to the trains!!!
