discovery flight
Definately choose the discovery flight. It costs a couple extra pounds, but it worth it! You get to skip the line and you get a guide in the pod with you, pointing out all of the sights! When we went on it, there were only 4 people, plus the guide! Considering they can hold about 25 people, there was lots of space to more around and the view was never blocked.
Make sure you go on a clear day so you can see everything. I'm not fond of heights, but I didn't find it that bad. It's not like a ferris wheel because there is no rocking motion - it's a very smooth ride.
Definately something to experience in London!