Wouldn't be too far for many to travel if its way east ?.
I don't think distance is problem
per se. I do see too issues:
1) The sheer scale - no airport has been closed and moved on this scale before - biggest to date being HKG? At time, Kai-Tak was just under the 30m mark, LHR broke the 70m mark last year, and T2/"Queens" is still a big shell.
2) The movement through central London. One advantage of airports in transport terms is that they are often moving people against the flows - ie morning departures are going away from the city centre. For FBI, you'd be bringing people by train into the middle of London, often making them change at already very busy stations, and then on out to the new terminal.
Perhaps the only viable option would be to have a 4 runway airport to the west or north west of London.
Between the 2003 White Paper and the weekly have-a-go-architect stories that have been all over the place this year, I think we've seen enough options. Part of the problem here is that architects are the ones who create the buildings, most of the ones that have commented on the airport issue, Foster included, really aren't up to speed on the masterplanning issues.