QUOTE'Ah but could you taxy a seaplane through the Thames Barrier?'
Not sure why you wopuld ever want to even if you could.. To make this sevice interesting the landing area needs to be close in to London. The water strip at London city is perfect and I have a suspicious feeling it is within the perimeter of the London City Airport property boundary.
I imagine it would work as it looks like there is about 1600mts x 100mts available.