For anybody who has the time to read it a great deal of information about how Eurocontrol has set about validating the integrity of ADS-B is available on their website. Just search for either Cascade or Cristal. The output tends to support Pelton Level and Bergerie1 rather than HD but it does agree with him that it isn't going to provide much benefit in the London TMA anytime soon.
A discussion on moving the ATC task from the ground to the air would be fun but surely should be in a separate thread as this one has already drifted a long way from its title.