Unless you have a ticket for the show the best place to see it is the East door to ExCel or from the bridge over Royal Victoria Dock.
Owing to the footbridge it will probably go through the swing bridge to the dock north of LCY. From there it can either take off from the water or be lifted into the airport like they do for maintenance in Corfu.
http://www.jetphotos.net/viewphoto.php?id=5618465