I always favoured an artificial island in the Thames Estuary (right in the middle, not the Isle of Grain), built Kansai style (or like the Chinese in the South China Sea, who are causing aggravation). It would be expensive but the HS2 train could be scrapped since people can get to Brum 20mins earlier by setting off 20mins earlier. It would need some type of high speed hover type transport to get into central London.