The trouble with mixed mode is:
(1) it does not address congestion and delays in the air (stacking) and on the airport (queing to take off);
(2) it does not provide a long term remedy to the UK's lack of hub capacity: LHR still needs two more rwys;
(3) it will lead to more noise for flightpath residents who lose their daily half-day of quiet, so politically unacceptable.
No argument there - I certainly wasn't proposing mixed mode as a solution, simply responding to the post that suggested it would already be in use today, if only FAA rules applied.
As I said in a previous post:
mixed mode is the ultimate lose-lose scenario
in that it upsets the most people for the least benefit.