Something to consider that I dont think has been mentioned yet is where is all the rock and cement going to come from to create the island ?
All the various options look considerably bigger than anything that has gone before, Hong Kong had the luxury of a mountain on site to re-profile. Any Thames based project is going to need Millions of tons of rock to create a platform to replace the mud of the estuary.
Very interesting report here which includes a short history of all the past proposals
http://http://www.smithschool.ox.ac....es_airport.pdf